Chevrolet: Oil pressure sensor ???, oil pressure gauge, size blazer

Question
We have a 1989 Full size Blazer 4WD with a 350. When you first fire it up when cold oil pressure reads around 60. After it warms up it starts too drop. After it is up to temp, the oil pressure will drop to 0 at a red light, but comes back up to 30 when driving. My buddy who owned to the truck says it's been doing that for years. I'm thinking the oil pressure sensor needs too be replaced. Where on the engine is it located ???  Also, the damn service engine soon light comes on after it gets warmed up. Lastly, when warmed up, if you shut it off, it won't start till cooled off sometimes. That one I have no idea why. Any thoughts ??  

Answer
Sorry ignore the last msg. I clicked the wrong button.

The oil psi issue is a serious one and not to be taken lightly. You need to remove the sending unit for the oil pressure gauge and install a mechanical gauge on it to make sure that you really have zero oil pressure. If you do, it may need a new oil pump, have worn bearings somewhere, etc.

The sending unit should be beside the distributor.

The no start issue is almost always a bad fuel pump.