Years ago my 90 NA 5 spd developed the same rear main crank seal oil leak. It was small, leaving a silver dollar sized pool of oil right under the bell housing after each drive. Someone here suggested replacing the PCV valves to get rid of the crankcase pressure that was pushing the oil out the seal.
I changed both PCV valves and the oil leak has been gone now for the last 10 years. I never did develop a clutch shudder because of the oil leak. Hope this helps!