The only one of the machines you mention that I am familiar with is the 15-91. On the 15-91, the only place you use 'grease' is in the grease cups that lubricate the motor. They are those upside down things under the hand wheel. They are a pain to get to, so are easy to procrastinate.
I would assume that grease is used in very few places on the other machines, but have not owned those machines.
bkay