Sanity Check...low oil pressure

[iamdub]

What wasnt answered here is what is considered normal for a high mileage engine. I had the P0521 code, and had about 20 psi at 2,000 rpm and about 10 psi at hot idle, would get up to about 25 psi at higher rpm. Running Mobil 1 5w30 in the 5.3 engine on a 2011 Yukon XL SLT with around 147,000 miles. Had the sensor and screen replaced and that didnt change my oil pressure readings. Now my P0521 is back and Ive been reading through the other possible causes such as the O ring. But what do others get for oil pressure on a 150k+ mile engine at hot idle and 2,000 rpm? Just trying to get a benchmark for what is considered normal.

There really isn't much to determine a benchmark based on mileage. Technically, it should have almost the same oil pressure as it did when it rolled off the assembly line for at least 200K miles. A lot, or very little could have happened to that engine in 147K miles that would affect the current oil pressures. You could say that a meticulously maintained engine with no mechanical faults would have high 30s and that would be normal. You could also say that a well used engine that was fed crap oil that wasn't changed as needed and had the resulting expected wear would be normal to have 20 psi at hot idle. OR, a perfectly maintained engine has a failed O-ring or faulty relief valve in the oil pump, resulting in lower pressure while a well-used engine has no faulty or ailing parts and has great pressure. There is no "it has X miles so the pressures should average around X and X".

For relevance, I bought mine at 147K miles. Up until I tore it apart at 200K, it always averaged 27ish-42ish depending on RPM, oil temp and type of oil I was running.

Yours has a problem and you can't simply relate it to the mileage. With no abnormal/excessive wear, at hot idle I'd guess yours should have around 25ish and 40ish at 2,000RPM. Start with the easy troubleshooting and work up from there. To test the O-ring, add 2 quarts to a full crankcase ("full", as indicated on the dipstick), park with the nose down on a slight incline and see if the pressures immediately improve. If not, your next step (after draining the two extra quarts!) is to investigate and likely replace the VLOM gasket.