Oil leak 1990 Prizim

If it is smoking out the exhaust pipe, that would indicate a major problem (valve seals, rings).
If it is burning oil (smoke coming from the engine compartment), then I would not drive it.
It is possible that the oil could come in contact with a heat source such as the exhaust manifold and cause a fire.
To replace the rear main seal you have to take the transmission out.
The seals get hard in time and the round spring that seals them against the rotating crankshaft can eat through the seal and scratch the crank (this spring is called the garter spring). If the scratch is bad enough the new seal will fail very soon.