My last 87 GN, a 7K mile car, leaked from the rear main seal, size of a 1 gal coffee can, why they leak? Crank flexing do to boost along with a cast crank. My fix was to install a Sabo Neoprene seal, it never leaked again, my built engine had the same seal installed and saw 25 + lbs of boost, no leaks from there. I bought a dye system leak detector that works on oil, coolant, and other oils. Yes leaks can come from other places like mentioned so have it checked out before replacing anything. Eastwood and Others sell such a dye check system. This is my experience with my car only and not intended to criticise anyone else. Good Luck on your Purchase. Gene