I have the same problem with my 87 and my guess is ( for my car):
I am recalling from memory the schematic, so I hope that I am accurate:
The oil level sensor has a little hole in it that the oil returning from the top end flows though, that completes a circuit in the sensor that turns off the dash light when enough oil is passing though it.
I am thinking that gunk has particially clogged this hole, so that when my oil gets hot, ( thinnner) it fails to register the signal, so my oil light stays lit, after the engine hits 80*C even though it is full.
I have plans to drop the pan and clean this little doo-dad soon.
Good luck with yours