I recently inherited my mom's 15-91. It has not really been used in years. However, my mom had it "serviced" every few years. Some guy would come out to the house and futz around with it for a while and give her a bill. Her husband said she had had a guy out not too long before she got sick. (10 years or so ago). The machine has not been used much, if at all, since then.
I brought it home and started cleaning and oiling it. The grease "pots" for the motor were almost empty. That means he didn't service it at all, and the guy who serviced it before that probably didn't do anything either.
bkay