Clean belts and clean clutch sheaves allow the sheaves to better grab the belt with less slipping, that’s less friction and heat. Logically we want to keep the drive belt and clutches free of oil, or any residuals that promote slippage instead of traction.
One question that often comes in is on cleaning drive belts. We’ve always followed the logic of no chemicals, using only a ScotchBrite pad in hot, soapy water. Soap? Something like Simple Green works well. You can scrub the belt clean, roughen up the glazing, get rid of the mold release (on new belts) and make them as good as possible. Do it to the sheaves as well, if needed, to clean away any petroleum or rubber residues that would take away from efficient power transfer. This is also a good time to closely inspect the belt for cord separation, layer separation, or any deformities in the power-transferring surfaces that are not supposed to be there.
Some guys swear by certain cleaning solvents, but we’ve always been of the following of not getting the petroleum-based chemicals into the drive belt and clutch sheaves. You should be able to remove any residues with a scouring pad and a good water-based soap cleaner and keep the system clean.