Forum

[Grunt]

I know people have oil "loyalty" and I did (back in the day - Castrol GTX!) but now I have a question. My Classic did 400 mmiles before a class "c" befell it and the dealer used Silkolene Pro4 fully synth. Fine. So I bought 5le. Now with a another new bike from a different dealer who uses Motul fully synth. As both types of oil are high spec 15W50 fully synth are they then compatable for me to do the regular top up bit? OK - stupid question but I have to ask just in case. TA

[zonggong]

There is a lot of experience and opinions on engine oil. Check out some of the other threads. I think there is a very slim chance that the oils are not miscible. Manufacturers would be silly to have products that do not mix with others, however, it is still possible. It is not only the base oil (synthetic, mineral etc) that may not be miscible but could also be the additives that the manufacturer uses to give the oil it's characteristics. These can be up to 20 - 25% by volume and can be significant. If you are concerned then do a small test. Put a 50/50 mix in a clean container (shake it up) and leave it on a windowsill (in the sun if possible) for a week. After this time is there are any deposits, separation, layers, chemical reaction, coagulation, or anything where the mixture is not' clear and bright' then the oils are probably not miscible. A lab test for miscibility is a refined version of this. If you are topping up then you may wish to vary the ratios for the test to say 10/90 as different percentages can also make a difference.

[Revband]

A very good and comprehensive answer from Zongzong, now my short answer. Millions of users have mixed oils for many years without any problem.

[ogri]

Revband, that's only the case if you're adding like to like, no?

[Revband]

Ogri, both the same spec just different makes?. I see no problem must have been done millions of times around the world.

[zonggong]

There is a small chance (< 5%) that the oils may not be miscible. There are many different types of 'synthetic' base oils and some don't mix with other types. Also, the additives used may not be entirely compatible or change the 'balance' of the product and this could result in some sort of deposit or drop out from solution. This could be even if the spec is the same. However, as Revband says lots of people do it without having a problem and if the oil is for an RE, well that is a very basic engine and as long as there is oil to run around the bearings it probably doesn't matter too much.