The machine it self doesn't take any grease, just oil. The motor takes grease, but if you apply through the grease ports, the modern synthetic greases don't wick into the bearings. There are one or two alternatives especially made for the old machines, or you might have to apply directly to the bearings. Some later motors were lubrication free. You can use grease on treadle irons, but only if you take it appart.
All the cast iron straight stitchers were flat beds. You need to fold down the legs and sort of work around the area by turning the bulk of fabric. Some people keep a freearm model for this reason, but mending pants should generally be fine on a flat bed. It has a lot to do with getting used to it, my suggestion is to persevere.