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Bike and car oils - that old chestnut !
Normal mineral car oils are fine ( assuming of course that you don't
have a bike that needs synthetic, ie KTM )
Problems start when you get onto the semi synthetic oils - like GTX
Magnatec, or fully synthetic oils like Mobil 1 etc. These are
designed for car angines and as such have special additives (
friction modifiers ) in them, here's a copy of a post I did on
another site that explains :
Yes, there are various types of friction modifiers (and
even "ordinary" additives like detergents and dispersants can affect
friction to a greater or lesser extent) so the key is choosing the
right ones
Car oils like GTX Magnatec / Mobil 1 are formulated to get the best
effect (eg. lowest friction or extra protection) in engines without
wet clutches or other friction dependent devices.
Motorcycle oils are formulated to get the lowest friction and best
wear protection, but still maintaining adequate clutch performance.
To do this properly you have to have access to various tests which
allow you to accurately measure clutch performance. There is no one
test that gives you all the answers when it comes to wet clutches on
motorbikes because it is not just slip you are trying to avoid, it is
also excessive drag. Few
companies have the resources to develop these tests, but Castrol is
fortunate to be one of them. The added difficulty for oil formulators
is that some bikes are more prone to clutch slip than others, and
multiplate bike clutches can be very variable, so that even
two "identical" bikes can be different in terms of clutch
performance. Then, of course, some riders are harder on clutches than
others. Therefore one rider could use a car oil in his bike without
problems, whilst his mate with the same bike suddenly finds he needs
a new clutch. The life of any clutch can be expected to be determined
by the point at which it starts slipping.
How long it takes depends upon how big or "strong" the clutch is
compared to the engine (obviously the clutch needs to be strong
enough to transmit the engine's torque, but light enough to be
operated by the weakest user you could reasonably expect), how much
abuse it gets in it's life (600cc Supersport bikes can write off a
clutch in less than three racing starts), what oil is used and how
often the bike is loaded up with extra weight
like luggage or pillions
If you look at the major components of an engine oil, there is often
an extra factor that needs to be taken into account for motorcycles
when compared to cars:
a) Detergent: surface active and so can affect the wet clutch
friction performance of the oil and may not be designed to cope with
the temperatures in Motorcycle engines, especially air-cooled engines.
b) Dispersant: surface active and so can affect the wet clutch
friction performance of the oil. Also sludge is much more of an issue
in passenger cars and so the choice of dispersant and the treat rate
in a car oil will not be optimised for motorcyles. This means a hard
ridden bike could start buiding up deposits that degrade the heat
transfer from the pistons to the cooling system and ultimately suffer
from lost performance due to stuck piston rings. Especially if you
don't change the oil regularly because the additives deplete over
time.
c) Viscosity Index Improver: (in multigrade oils) can affect the
amount of clutch drag you get in cold weather and also needs to be
more shear stable
for motorbikes because of the integral gearbox
d) Baseoils: Use of higher quality baseoils allows the amount of
additives to be reduced, allowing friction performance and shear
stability (stay-in-grade) to be maximised. Also, some of the most
expensive
baseoils (esters) can reduce friction enough to cause clutch slip -
GTX Magnatec contains esters in its formulation
e) Anti-foam: can affect the clutch friction performance of the oil
(even though the additive is mixed in at a tiny percentage)
f) Friction modifiers: by definition affect the friction performance
of an oil and so need to be carefully matched to the application.
So, by using a car engine oil, you could be using an oil with
additives that are not able to cope properly with the extra demands
of a motorcycle engine that has a wet-clutch and integral gear-box.