WHAT THE CLUTCH NEEDS
Motorcycle clutches are like the engines they serve - compact, high performance and highly stressed. Instead of one large clutch plate, they use a series of smaller ones, separated by rings, with springs squeezing them together.
When the clutch lever is pulled, the plates are pushed apart, allowing them to slip - breaking the power connection between engine and gearbox. As the lever is released, the springs push the plates together again, allowing friction to feed power through the gearbox to the rear wheel.
Whenever that happens, those slipping and gripping plates generate an enormous amount of heat, so the primary purpose of the oil here is to cool the clutch and to prevent the bending and burning that would otherwise damage the rings and plates.
It’s a delicate balancing act between friction and slip - with the oil caught in the middle.
If that oil is friction modified - as many fuel efficient car oils are - it will struggle to maintain grip between the plates and rings and can lead to clutch slip: the driving components spin without transferring all the power through the clutch to the road.
The rider may not know why it’s happening - but he’ll know the clutch is slipping because he loses acceleration - probably accompanied by the smell of fried clutch plates.
If the oil friction is too high, the clutch may snatch or grab when engaged giving poor power take up - either when pulling away or during gear changes. Again, the rider will feel something wrong - because the machine will lurch or jerk as the clutch is let out. If the viscosity of the oil is too high, the clutch may drag - failing to disengage with the lever pulled in. This is a particular problem when the engine is cold and in motorcycle clutches where the operating movements involved are quite small.