The place to start is by having the actual oil pressure checked with a mechanical gauge. There were some problems with the sending units in the mid '90s, and they would cause what you're describing, but if you already replaced it, that would have solved the problem. To double-check, there should be two terminals in the old sending unit. One is for the gauge and one is for the warning light. If yours just had one terminal, you might have replaced the wrong one. Look for a second one in the same area.
If the pressure really is low once the oil gets hot and thins out, it's usually due to worn engine bearings. Oil is still getting to them and doing what oil is supposed to do, (isolate moving parts from each other), but it's running out too easily. It very likely is still possible to have new bearings installed without having to do any other major engine work. Once the knocking starts, the crankshaft will usually be damaged and need to be replaced. At the very least that requires removing the engine.
The owner's manual will list the oil viscosity recommended for the temperatures in your area. A higher viscosity might bring the pressure up a little. I have an old rusty trusty '88 Grand Caravan with almost 400,000 miles and I've been running the cheapest 10W-40 I can find for many years. It originally called for 10W-30 which is a little thinner. Even the cheapest oil is made by a name brand manufacturer. It's just packaged for other retailers. All oil is much better than what we had available a few years ago. The detergents and other additives are also much better.
Sunday, April 8th, 2012 AT 10:13 AM