When I first crank my truck, oil pressure is "normal" (between half and 3/4 the way up on the gauge). Soon as the engine warms, it drops to between 1/4 and 1/2 on the gauge depending on the RPM's. I am running 5w30 in the truck and just did a head gasket job where I replaced the stat, water pump, and most of the gaskets in the upper half of the engine. Truck runs great other than the low oil pressure - no valve chatter or anything like that that I can hear. I'm thinking it could be the oil sending unit as it still has some pressure... but part of me knows that it very well could be worn cam bearings or something of that sort.
So pretty much my question is what is the best way to diagnose the cause of my low oil pressure, and if I can't fix it would it be safe to drive the truck on 1/4-1/2 pressure on the gauge? I should probably mention I have no idea what weight oil was ran in the truck before I did the head job - it was extremely sludgy and black. The truck sat for 6 months before.
My car has 155000 miles.
My car has an automatic transmission.
With the weight of the oil, 5w-30 is preferred in climates that see temps below 0 degrees, and 10w-30 for 0 degrees and above. With that said, it is not unusual to see a drop in oil pressure when the engine warms up. The oil will "thin" a little with the added temperature.
Personally, if I have a question whether the oil pressure sender is bad, I tend to replace them, as they are cheap and replacing it negates any questions of its ability to perform.
If the pressure still appears to be low, I would suggest removing the oil pressure sender, and installing an oil pressure gauge. The specification I’m finding for your vehicle is: 56 lbs @ 3000 rpm.
If you need some assistance, consider YourMechanic, as one of our mobile technicians can come to your home to diagnose any issues and replace your oil pressure sender as necessary.