I woundn't recomend grease or motor oil because it'll break down the rubber. It also seems like it would allow the tire to spin on the rim after mounting. Use lub they sell for mounting tires. If the rims are too rusty, it's time to replace them for safety reasons.

14" tires? Where did you find them? How old are they? I'd be wary of tires over 7-8 years old.