I doubt you have major problems. I'm not sure 20-30rpms is enough to worry about. On the variable speed drives oil seems to bleed back, but in that case rpms usually don't recover. Manual tells you how to set tension properly for the belt. There's two grease nipples to lube the pulley movement but bearings do not get greased. If you've got more than 1300 sep hrs I would likely remove vs pulley assembly and check or change the one bearing.

 

You’re only talking 5% or less, just normal machine component loading.

 

What combine are you running? Feeder house drive type? Five speed or high torque variable speed?
the five speed really don’t give much problem but the high torque or even the heavy duty variable speed with the spring on the reverser sheaves give problems if they haven’t been serviced And properly adjusted.

 

9610 8 row head

just have the variable speed drive

 

Is your lower drive belt glazed? Can you dig your nail into the side of it easily? What is your slowest back shaft speed? Should be 500-520 with no load. The belt should be adjusted that it should be as parallel as can be on the lower sheaves. Also it could be in the upper sheaves. There is a set of cogs inside the sheaves that strip out and cause belt slippage. I’m just outlining the things that cause slippage. Your 20-30 is probably in the safe range. I would check the belt and I’d it had been glazed then pull the upper sheaves apart to see if they are ok.