Mybeweno,

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