I have an 85 deville rwd bad oil leak at rear of motor. I've replaced the pan gasket to no availe what's next?????

I'm not familiar w/ the RWD setup and the 4100 except for Eldos, but...

Are you sure its not coming from somewhere 'above'? Like the rear intake
manifold gasket, or the oil press sending unit? Those are two LIKELY problem spots to check out.

Oill filter adapter is a known leak source on 4.9 . I do not know if it is the same on 4.1.