I have a '93 240 with 360k miles. It looks an runs very well except for one thing. I installed an aftermarket oil pressure gauge because the oil light flickered once during idle, and here's where it gets scary. When the car is cold, the pressure is around 50 psi at idle and near 100 psi when winding out the gears. When it's warm, the pressure is about 5psi at idle and 30 psi at cruising speeds. I have always used Castrol GTX 10W40 oil. After reading many posts on this forum and elsewhere, it was the consensus that it could be the oil pump, the "O" rings on the tube or possibly the main and rod bearings.

I removed the oil pan (a real pain under the car with oily mud falling on you) and changed the oil pump and "O" rings. I used plastigauge to measure the bearings, and they were in spec, but since I already bought new bearings, I changed them anyway. Yes you can turn the crank and "spin" the old bearing out and "spin" the new bearing in.

The problem is still there. The engine is very quiet (no rapping) even when the pressure reads near 0. My question is does anyone know the oil circuit? Where does the oil go after the main and rod bearings? I know it somehow goes up to the top to lubricate the cam. Could there be excessive clearances up on the top of the engine causing this drop in oil pressure. I am now using 20W50 even in the winter and now it's 12 psi at idle when warm.

Any ideas short of removing the engine and rebuilding it would be appreciated.