I have an '87 F-150 with a 4.9L 300 inline 6 in it. It has ~111k miles on it. I'm having oil pressure issues. We've replaced the pump, the oil sending unit, changed the oil numerous times, changed oil pan gasket, cleaned the pickup screen. It still has low oil pressure.

When I start it up it's at or a little below normal. After running for about 10-15 minutes it drops to right above the red zone. If I let off the gas and let it idle the oil pressure is completely non-existent, but when I step on the gas and it goes above 1,000 RPM the oil pressure picks back up above the line. If it goes below 1,000 RPM it drops completely to nothing at least what the gauge reads. It has a mechanical gauge as well as the factory gauge and they both say the same thing. The motor doesn't knock at all.

Any ideas what the problem is and how to fix it?

 

Low oil pressure is also a sign of warn bearings.

 

could be the sending unit is bad. 111k isnt really allot for those motors. I'd try replacing the sending unit. cheap part.

 

X2 on the worn bearings. run the thickest oil you can find lol.

 

Running 20W-50 in it now. Which bearings would be more likely to wear; cam, crank, rod, or mains?

 

My '86 Jeep cj7 does the exact same thing. I've never worried about it. Runs just fine the way it is. I even bought it like that. I've been told that it's the rings.