I've had this truck for a couple years, and barely use it. In December it started with major oil pressure drop once run for awhile. Especially at idle. Hadn't given me a problem before so never changed thermostat, temp gauge just never worked. So I decided to dive in and change some parts. New oil pump and pan liner, check. Some change, but minimal. Still problematic. Overheated and burned off the ECT sensor, started next day after it cooled down. Changed thermostat, flushed coolant system best I could but it was filled with stop leak🤦. Changed ECT sensor, flushed radiator once again. Filled it up with coolant. If I run heat it will either work as coolant moves, or won't work and overheating happens. I guess I'm going to try to change water pump, then radiator. Oil cooler lines are bleeding through, not too familiar with an oil cooler. Any help would be appreciated, parts are pretty cheap, it's just everything else is expensive right now so I am unable to just change everything at once.

 

Just speculating here, but could the external oil cooler be clogged and causing a restriction in the flow of oil through the system? I also have an old GMT400 as well (1996 C1500 5.0L) and read a lot about folks bypassing the oil coolers on these older trucks to get rid of leaks and other problems. My 1996 doesn't have an oil cooler, just the stock transmission cooler, but I've thought of adding an aftermarket oil cooler if I ever decide to tow with the old truck again.