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Author Subject: Which Oil?

Seasoned Pro

Location: Nottingham

Registered: 15 May 2004

Posts: 11,299

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Post #1
A question that always sparks debate, I thought it best to put it in the FAQ. Also found a load of Guy's threads and thought we could stick them in here so they don't get deleted Happy

The Basics

I had here an explanation of the differences between different weights of oil and their advantages and disadvantages.

But it confused the hell outta me and it was wrong.

So someone find a good link, and I'll stick it on here Big grin

Semi-Synth oil is a natural oil which has been treated to make it suitable for engine lubrication. Fully Synthetic Oil has been manufactured in a lab for the specific purpose of being a lubricant in car engines.

The short story:

Peugeot recommend 10w40 semi synth for our engines. If the oil is changed regularly the engine will last well into the 100,000 mile mark with no problem. However Semi Synthetic oil breaks down and doesn't protect as well over time. That's why it needs changing regularly.

Some people use Fully Synthetic oil because it doesn't brake down in the same way. This means it will better protect the engine over time and the oil doesn't need changing as often (say, every 12k miles as opposed to 6k miles for semi synth)

However, there are a lot of 'fake' Fully Synthetic oils about and real Fully Synthetic oil costs around £40 for 5 litres. For this reason most people stick to Semi Synth as it does the job very well, its very cheap and its whats recommended by the manufactuer.

Also Fully Synthetic oil tends to come in 5w40 or 10w50 flavours. As I understand it, 10w50 Fully Synthetic oil better protects the engine when hot, and is more suitable for cars that are going to be driven hard or on track days, presumably because the engine will spend more time at high rpm's.

Its entirely up to the consumer at the end of the day. You pay for 'theoretically' better oil, just as you pay for 'theoretically' better petrol when you fill up with Optimax. The choice is yours.

Oilman's wise words Wink

Do I need to flush the engine when changing from Semi Synth to Fully Synth oil?

When synthetic were in their early stages around 30 years ago there could be issues of changing from one to another. These days it is just an old wives tale, follow this procedure and there will be no problems.

Flushing Procedure (Don’t use flushing oils or additives)

1. Warm up engine to get oil circulating
2. Turn off engine and drain old oil
3. Fill with new oil to minimum (you will be wasting this)
4. Warm up engine to get oil circulating
5. Turn off engine, drain new oil and change filter
6. Fill to marker on dipstick

How often should you change oil?

(Regarding Fully Synth) There really is no problem leaving it in for 12K, oil companies want you to change your oil as often as possible so you buy more, little secrets about the stability of ester based oils are kept quiet, Ester are used in the aviation industry for many years because of their resistance to shearing down, you do not need you engine to seize at 20,000 feet.

Have a read of this I am going to do some typing for you!


The necessity of 3,000 mile oil changes is a myth that has been handed down for decades. Well it’s good for the oil companies as they sell more oil!

Synthetic oils are much better value for money than petroleum oils, you just have to know
how to use them to your benefit.
If you're one who thinks that synthetics are just a marketing ploy to make more money off the same bottle of oil, I hope you'll take the time to read through this information and judge for yourself. The whole point of using a synthetic oil is peace of mind, knowing that you can trust the oil in your car to protect the engine.
There are only a few basic reasons why it is necessary to change your oil, and they all, in the end, have to do with decreased protection of your engine and decreased performance. If these elements can be minimized, then there will be little or no reason to change the oil.

What causes the oil to breakdown?
Firstly, all oil breaks down. That generally will include basestocks and additives (actually additives are really "used up" rather than broken down.

Without looking at performance characteristics, the most significant difference from one oil to another is how quickly breakdown occurs. Although there are many factors that contribute to the breakdown of an oil, temperature is one of the most important.
Petroleum oil begins to break-down almost immediately. A high quality synthetic, on the other hand, can last for many thousands of miles without any significant reduction in performance or protection characteristics. Synthetics designed from the right combination of basestocks and additives can last indefinitely with the right filtration system.
The first major difference between petroleum and synthetic oil is heat tolerance. Flash point is a technical specification referenced by most oil manufacturers which is an indicator of
heat tolerance.
The lower the flash point of an oil the greater tendency for that oil to suffer vaporization loss at high temperatures and to burn off on hot cylinder walls and pistons. This leads to oil thickening and deposit build-up on critical engine components. So, the higher the flash point the better. 400 degrees F, is the absolute MINIMUM to prevent possible high consumption and oil thickening due to burn-off.
Today's engines are expected to put out more power from a smaller size and with less oil than engines of the past. Therefore, the engines run much hotter than they used to. That puts an increased burden on the oil. Synthetics are up to the task. Petroleum oils are a little overmatched.
Nevertheless, even though synthetics are MUCH less prone to burn-off than are petroleum oils, there is still a small amount of burn-off during extremely high temperature operation.

The manner in which petroleum and synthetic oils burn off is important. As a refined product, petroleum oil molecules are of varying sizes. So, as a petroleum oil heats up, the smaller molecules begin to burn off. Deposits and sludge are left behind to coat the inside of your
engine. In addition, as smaller particles burn off, the larger, heavier molecules are all that is left to protect the engine. Unfortunately, these larger particles do not flow nearly as well and tend to blanket the components of your engine which only exacerbates the heat problem as friction builds-up.

Synthetic oils, on the other hand, because they are not purified, but rather designed within a lab for lubrication purposes, are comprised of molecules of uniform size and shape.

Even if a synthetic oil does burn a little, the remaining oil has the same chemical characteristics that it had before the burn off. There are no smaller molecules to burn-off and no heavier molecules to leave behind. Moreover, synthetics contain far fewer contaminants than petroleum oils since they are not a refined product. As a result, if oil burn-off does occur, there are few, if any, contaminants left behind to leave sludge and deposits on engine surfaces. Obviously, this leads to a cleaner burning, more fuel efficient engine.
It is also important to note that synthetics do a much better job of "cooling" engine components during operation. Because of their unique flow characteristics, engine components are likely to run 10 to 30 degrees cooler than with petroleum oils. This is important, because the hotter the components in your engine get, the more quickly they break down.

What is Additive Depletion?
Additive depletion comes into play when discussing oil drain intervals. If additives
are depleted, the oil cannot effectively do it's job. So, the oil must be changed.
It is true that the additives in many oils begin breaking down after only a few thousand miles. What needs to be recognized is that there are different quality "grades" of additives just as there are different quality grades of just about any other product that you buy.
Many oil companies are using the same additives in their oils as all of the other companies because they are cheap. That's why the oil costs less. You get what you pay for!

If they were willing to spend the money on top-quality additive packages for their oils, every synthetic on the market would be recommended for extended drain intervals, and they would all be more expensive. The technology has been around for years. The problem is that oil companies make more money selling a cheaper grade oil and making sure that you change it more often. So, what do the oil additives in your oil do?

Viscosity Retention
Additives are used to maintain a stable viscosity over a wide temperature range. Synthetics need less of these additives than petroleum oils do because synthetic basestocks maintain a fairly stable viscosity by themselves. Also, the additives that are used are more stable than those used in petroleum oils.
As a result, petroleum oils must be changed often because they quickly become unable to retain the viscosity levels necessary to protect your engine (their high temperature viscosity drops off).
Synthetic oils don't really have that problem because both the basestocks and the additives are more stable.

Contaminant Control
Additives are also used to keep oil contamination in check and to keep it from damaging your engine. These additives keep potentially wear causing contaminants suspended and contained in your oil so they don't cause excessive wear or deposit build-up within your engine before your filter can remove them.
Synthetics generally have higher additive treat rates than petroleum oils (in addition to using
higher quality, more expensive additives), so they can perform this contaminant control function for a much longer period of time than a petroleum oil can.

How Does Oil Contamination Occur?
Of course, additive depletion and degredation of oil basestocks are not the only things that cause an oil to need changing. There is also the issue of contamination.
Oil will be contaminated in three major ways. One will be through debris that comes in through the air intake. Once it makes it through the air filter, it ends up in your oil. Once in your oil, it starts damaging your engine.
The second source of contamination will be metal shavings from the inside of your engine. The lesser the quality of the oil, the higher percentage of these shavings because there will be more wear inside the engine.
The third source of contamination will be from combustion by-products. Combustion by-products will generally raise the acidity of your oil, which causes corrosion in your engine. In addition, they will be left behind as the engine oil burns off and will collect on the inside of your engine as deposits.

What viscocity should I use?


Let's sort this out once and for all, have been doing loads of research and the listed viscosities are as follows: (figs all DegC)

Pre 2000

-30 to 25 = 5w-30
-30 to 40 = 5w-40
-20 to 25 = 10w-30
-20 to 40 = 10w-40
-20 to 50 = 10w-50
-15 to 40 = 15w-50
-5 to 50 = 20w-50

Post 2000

-45 to 45 = 0w-30 / 0w-40
-30 to 45 = 5w-30
-30 to 50 = 5w-50
-20 to 45 = 10w-30 / 10w-40
-20 to 50 = 15w-50
-5 to 50 = 20w-50

The book says for all year round use UK 5w-40 or 10w-40 and this is what I would stick with but bear in mind 5w-40 will circulate more rapidly when you turn the key so it gives better cold start protection.

Now to clear up some other things:


More expensive, better protection and essential if you are competing in your car or running higher BHP etc as they handle the higher temperatures better than petroleum oils.

Using synthetics in a road car is personal choice, they can be changed far less frequently and give superior protection to other oils ie wear, heat and cooling but you need to decide whether you can afford them.
Are they worth it? YES!

Here are some common synthetic myths:

The Myths regarding Synthetic Oils

The benefits of Synthetic motor oils have been much debated over the last 10 years and misinformation is rife, particularly on the internet.

There are many so-called experts out there who should know better but hopefully this post will shed some light on some of the most common myths.

What are Synthetic Oils?

Synthetic Oils are fuel efficient, more fluid and resistant to thermal breakdown, they are constructed in laboratories using basestocks and special additive packages. They are specially formulated to meet and perform to standards set by API and ACEA as required by OEM’s.

Synthetic motor oils damage seals.

This is untrue. Why would lubricant manufacturers build products that are
incompatible with seals. The composition of seals present problems that all types of oils must overcome.

At the end of the day, it is the additive pack in the oil that counts. Additives are added control the swelling, shrinking and hardening of seals.

Synthetics are too thin.

This is untrue. In order for an oil to be classified in any SAE grade (0W-40, 5w-40,10W-40 etc) it has to meet guidelines with regard to viscosity or thickness.

For example, any oil with a viscosity of 10W-40 has to operate at -25 degrees centigrade and 100 degrees centigrade to pass these tests or it cannot be rated as a 10W-40.

Synthetics mean higher oil usage.

This is untrue. Synthetic motor oils are intended for use in mechanically sound engines, that don't leak oil. In these engines oil consumption will actually be lower because of the lower volatility of Synthetics. They also have better sealing capabilities between piston rings and cylinder walls. Synthetics also have better oxidation stability. (They resist reacting with oxygen at high temperatures)

Synthetic Oils are not compatible with other oils.

This is untrue. The synthesized hydrocarbons, polyalphaolefins, diesters and other materials from high quality basestocks are fully compatible with other oils.

It is best to stick to the same oil for topping up that you have in the engine. It is best not to mix oils, as additives are blended for specific oils. When different oils are mixed additive pack balances can be upset so for the best performance, it’s better not to mix them.

Synthetic Oils produce sludge.

This is untrue. It is a fact that they are more sludge resistant than other oils as they are better at resisting high temperatures and oxidation. Because Synthetic oils have higher flash points, they withstand evaporation better leaving less deposits.

Synthetic oils can't be used with catalytic

This is untrue. There is no difference between synthetic and other oils with regards to the components. Neither will damage catalytic converters.

Synthetic oils can void warranties.

This is untrue. No major manufacturers specifically ban the use of synthetic
oils. More and more new performance cars are factory filled with Synthetic oils.
Vehicle warranties are based upon the use of oils meeting specific API Service
Classifications and recommended viscosities.

Synthetic oils will last forever.

This is untrue. There are some people that believe that synthetic basestocks themselves can be used forever. However, it is a well known fact that eventually the additives will break down and cause the oil to degrade. The additives in the oil are effectively “used up” by moisture, fuel dilution and acids. Regularly topping up the oil will help but sensible oil change periods are recommended.

Synthetic oils will protect an engine for far longer periods than non-synthetics.

Synthetic oils are too expensive.

This is untrue. It has been proven through testing that Synthetic oils do have longer drain periods and provide better fuel economy. Add this to reduced engine wear and better reliability then do the maths. They are in reality better value for money than other oils.

Hope this clears up a few things.

Should I use Additives like Slick 50?

Don't use them, use a decent oil!

General Remarks on Chlorinated Additives.

A number of ‘add-on’ additives intended to improve the performance of commercially available automotive lubricants have been marketed in recent years, under such names as ‘Xxtralube ZX-1’, ‘Metol FX-1’, ‘PPL Anti-Friction’ and ‘Activ-8’.All such products share the following characteristics with ‘X-1R Friction Eliminator’:-

1)They all contain chlorinated paraffin ‘exteme pressure’(EP) compounds first used in the 1930s in heavily-loaded industrial gearboxes, and in some automotive transmission applications, mainly hypoid gears.

2)They all corrode copper-based alloys at moderate temperatures, easily exceeded in all engine, and most transmission applications.This problem was recognised in the 1930s, and chlorinated compounds were never used in transmissions with bronze bearings or gears. No responsible manufacturer ever suggested using them in engines where their increasing activity at high temperatures could lead to piston ring corrosion and bore glazing. (For the same reason, modern ‘hypoid’ additives are not used in engines, even though they are much safer than any chlorinated additive.)

3)X-1R Friction Eliminator and its clones are based upon very outdated technology, which was abandoned by responsible lubricant manufacturers for automotive transmission uses in the 1950s. Chlorinated compounds still find applications in metal working, but their use is on the decline because of health and safety considerations.

4)When burnt, chlorinated paraffins produce corrosive hydrochloric acid, and organo-chlorine compounds including the highly poisonous phosgene gas. Apart from these corrosion and health hazards, with petrol engines the deactivation of exhaust catalysts is also a problem.

5)Unfortunately, these additives give spectacular results in simple EP test machines such as the ‘Falex’. As a marketing ploy, a demonstration of this type looks impressive to those not aquainted with the above facts. Also attractive is the low cost of chlorinated compounds, allowing profits of several thousand percent to be made.

Guy's Budget Oil Recommendations

Have you considered Motul??

They do some fantastic oils through the range, and they are French to go nicely with your car.

Couple of options.

Motul 8100 Xcess 5w-40, this is a true synthetic made from pao/ester base stock and very good @ £28.99 for 5ltr.


Motul 6100 Synergie 10w-40, this is a blend of hydrocracked mineral oil and pao synthetic so it is above the average straight hydrocracked oils, and good value @ £19.99 for 5ltr.

Amsoil do make some good oils, most of them are average but the top end ones are OK, being American they are well over priced by the time they hit our shores, like redline.

There is a problem with American oils, the Americans do not have access to good quality VI Improver, this is the molecule that keeps the oil in grade through the temp range.

So the oils are good, but for a short while as the VI improver breaks down. Think how hard the average american engine has to work!! their oils are designed with this in mind.

The Esso semi syn is fine, but for a race car I would be looking at a more shear stable oil, one that can deal with the temps and cavititaion caused by track use, this is when ester/pao oils come into there own, so for that car I would suggest an oil like the Motul 300v or the Silkolene Pro range.

Tech specs here http://www.opieoils.co.uk/lubricants.htm

Mobil 1 is good stuff, leaders in their field of straight pao synthetics, trouble with them is you pay for a brand, but we do that with many things in life.

GTX, hhhmmm basic, much better around.


Guy Wink


always in stock, just PM me to order
Posted 11th Feb 2005 at 12:25

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