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TT Ade said:
Hi All

Which is the higher grade?

How do the two sections break down, what do they refer to?
The 0W or 5W figure refers to the viscosity at zero deg. C whereas the other figure is the viscosity at 100 deg C. The bigger the number the higher the viscosity but it refers directly to an SAE oil equivalent standard.

e.g. 0W-40 means the oil behaves like an SAE 0 oil when tested at zero deg C. and like a SAE 40 oil when tested at 100 deg. C.

Normally oil will get thinner at higher temperature and various tricks are employed to stop this happening, such as adding viscosity improver - long chain molecules which are coiled up when cold but open out into long chains when hot to thicken the oil.

The "W" refers to "Winter" and in cold conditions the lower the viscosity the better for cranking the engine over. At high temperatures you want to maintain a higher viscosity. Hense the term "multigrade" to suit different conditions. It used to be the case that oil was changed in winter and summer but multigrade oils did away with that.

More info here: http://www.upmpg.com/tech_articles/moto ... index.html
 

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Discussion Starter · #3 ·
Cheers, John. Great explanation, thanks.

So, I would be right in saying that the 0W/40 would provide protection over a greater temperature band. The two specific oils I have been looking at are:

Castrol Edge: 5W/30
Mobil 1: 0W/40

I generally buy Mobil 1, this time though the Castrol Edge looks like it has Audi backing, it's full of Audi on the label so no doubt a bit of banter going on between the two at the moment. Think I'll go for the Mobil 1.
 

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The viscosity rating is by no means the most important thing.

There are various measured parameters, such as "shear stress" which directly relates to the ability of the oil to hold together in a film before it splits under increasing pressure and allows metal to metal contact in a stressed bearing for example. Synthetic oil tends to be about three times better in this respect than a mineral oil with the same identical viscosity rating.

Stability of the oil is also important. I mentioned viscosity improvers, earlier. It's possible to make a 0W-50 oil with the addition of viscosity improvers but viscosity improvers have a limited life. Those long chain molecules get chopped up with use and the viscosity of the oil at higher temperatures will drop, so at the end of its service life the 50 may end up more like a 30 or 40 for example. The polymers can also undergo thermal degredation and form deposits in the engine. A better oil has less need for viscosity improvers and will have been achieved by blending synthetic base stock oils.

It's also not necessarily true that a higher viscosity is better in a high revving engine. If the oil is thinner it will tend to flow better and reach and replenish oil in all the bearing nooks and crannies, whereas a thicker oil may struggle to replenish some areas - after all it's pumped at the same pressure once the oil pump's pressure relief valve starts to blow off. Formula 1 engine oil tends to be 0W-20 for this reason.

Other things to consider are things like detergents, which are added to keep the engine clean by keeping waste solids in suspension, and waxes which coat bearing surfaces.

Synthetic oil is better in general than mineral oil because the base stock oil doesn't degrade, break down and form deposits and sludge.

Then there's the type of synthetic oil. Many are not really synthetic PAO (Polyalphaolefin) but highly refined hydrocracked carbon mineral oils that are allowed to be classified as synthetic by a loophole. They are not as good. Yes it's a bit of a minefield :roll: .

As far as I know, based on what I've read, the best oils are synthetic Ester based oils as used in aircraft engines. The Ester oil molecule is electrally charged and adheres ot metal surfaces. This gives the oil the ability to ensure lubrication happens at engine startup before oil pressure has built up - this is where most engine wear usually occurs.

Motul make the only Ester based synthetic oil which is suitable for use in the TT as far as I'm aware. It conforms to VW spec 503.01 which is another thing you should ensure when choosing an oil for the TT. More information here: http://www.********.co.uk/ttforumbbs/vi ... hp?t=81979
 
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