I have an 03 1500 5.3 silverado with 328k miles on it. About a year ago, started having issues when on longer drives(30 minutes or more) where it started sounding like there wasn't a drop of oil on the head/valves. From cold start, every time I drive it, there is a gradual decrease in oil pressure. Starts at around 60 and drifts down below 40 and will eventually bottom out around 20. This usually takes around 30 minutes, unless I really put pressure on the motor....passing another vehicle or just accelerating overly fast, and then the oil pressure drops of much faster. I don't drive the truck that much any more and sometimes oil changes are 6 months apart but when I changed the oil, I noticed a good bit of sludge. So, I dropped the pan and found I had 4 inches or so of sludge/buildup in the bottom of the pan. After cleaning the pan, the head/valves and replacing the pickup, strainer and all O-rings and gaskets, I refilled with oil and I still have the same issue. I was running Havoline Full Synthetic(High Mileage) but I went with Mobile 1 Synthetic(High Mileage) now, not sure if its related or not but wanted to go with something different.

One last thing, when the pressure falls to the point to where I start to hear the valve tap, if I turn off the engine and restart it, I get pressure back up close to 60 again but it immediately starts drifting back down and doesn't take nearly as long to begin tapping again (10 minutes or less).

Thanks for any help

 

At 325K you'll likely need a full rebuild or a crate motor. The only motors that will pull 500k in a pickup are diesels and some old straight 6 gas-motors _car_ motors like the old Chevy 250 cubic inch. High-mileage oil is a scam. (75,000 miles is high miles?) At most they have seal expanders for leaks, otherwise you are wasting your money.

 

Update: Problem solved, thank God. Replaced the oil pump. Running like new.

I've asked many mechanics and none of them had ever seen this type of activity with oil pressure. Hope this helps someone in the future.