I have an 86 GXL that is leaking oil my dad dropped by his mechanic to take a look at it and the guy said he was pretty confident it was coming from where the transmission meets the engine. The oil only leaks when i drive the car not when it is sitting and i have not noticed any change in how the clutch works anyone have any idea what is leaking/ the cost and amount of work involved?

 

Could be the

Oil Banjo bolt on Rear Iron, on the bottom
Oil Pedestal O-rings, where the oil filter mount attaches to the block, there is 2 o-rings under it.
Rear Main seal, these don't go out very often.
Oil Pressure sending unit, rear Iron under the oil Pedestal.

 

A "mechanic" should not be "pretty confident", he should be absolutely certain where an oil leak comes from. It takes absolutely no effort or skill to diagnose an oil leak.

Degrease the engine fully and then run it while stationary. Observe where the oil is leaking from. Leak is now diagnosed.

The common leak points are mentioned in the post above mine.