I spoke with a friend of mine today who has 50+ years in the equipment repair business (not confined to just forklifts). He has seen wet clutches on all types of equipment with flat & wavy clutch discs. He's also had multiple oils used in these applications.

His 2 cents is that he has never personally seen an oil misapplication cause a sticking issue- it's always been a slippage issue. He has found that a wrong application of oil will cause a clutch to slip- not stick. Also- some discs are wavy, some are flat & he agrees that the wave is there to help with separation (I stand corrected).

His suggestion was to disassemble the clutch assy, make sure that the disc isn't binding on the input shaft & to make sure that the pressure plate was working correctly & not binding.

If the clutch assy passes these 2 tests, then assemble the clutch back together & measure the clearance of the disc to the pressure plate & flywheel while the clutch pedal is depressed.

I know you said that you can't see inside the bellhousing when the unit is completely assembled, you may have to take these measurements while it is partially disassembled.

Hope this helps