By Harold Sharon
I am very familiar with wet multi-plate clutches but these are general comments.
If you have a “grabbing” clutch with 90 Weight oil/grease, I suspect that the problem is not the oil. Are you sure that it is a multi-plate wet clutch?
Too heavy of oil can result in slipping or long engagement time when up-shifting or starting on a hill.
Too light of oil can result in chattering or grabbing.
Most clutches do not have exotic seal material and can use any type of oil such as ATF but many people use a mixture of motor oil and kerosene to meet their temperature dependent needs:
For COLD weather, as much as 50% kerosene may be needed, but less for hot climates.
The clutch will let you know if you are too thin or thick as per the above mantra. Avoid Extreme Pressure (EP) formulations since they may be detrimental to any bronze elements and will generally cause slippage.
The clutch works by squeezing out the oil between the plates until you have metal to metal contact. EP oils may never squeeze out.
Chattering may be a sign of worn splines, guides, plates, or release springs. But it could also be caused by varnish or gunk on the plates that keep several glued together all the time. Never discount the possibility of an improperly assembled clutch. There are a LOT of parts that have to be assembled exactly right for these to work smoothly.