Vintage Synth Repair

I'm not familiar with the insides of the optigan (I'm a mellotron
owner!), but it seems to me that the extra coating of rubber is
unlikely to be the sole cause of y'r pitch wobbles. 

try measuring the speed of the drive motor some other way than by
playing the machine, & see if the speed regulator circuit is behaving
itself. 
e.g. listen to it's pitch, or measure the off-load voltage supplied to
this motor, if it's DC driven... if it's an AC motor driven directly
from the mains, it may just be in need of a good clean.

you could clean this rubber off the brass roller without doing any
harm (I have seen similar in many tape decks) but I'm not convinced it
will make much difference unless this rubber compound is going gooey &
breaking down. you'd see other evidence of this on the rubber wheel
itself anyway.

duncan.