It very well could have low oil pressure. It's not the brand of oil -- Valvoline is top-rated oil. Low oil pressure is pretty common on these...but usually not till closer to 200k. The other reason (in some cases) is the timing chain plastic breaks up and clogs up the oil pick-up screen usually at sustained higher engine speeds. You'd probably hear louder chain noise if that was the case. But worn bearings is the usual culprit, possibly a weak oil pump. GM wisely use aluminum bearings. The best course of action to see what you have, would be to get an accurate oil pressure testing gauge 0-60...or something like that (notice mine below is a 0-30 calibrated gauge), remove the oil switch and see what the oil PSI is under the same (warm) conditions you determined earlier. Takes an 18mm adapter (buy off ebay). You can use a simple and cheap grease gun hose with a liquid gauge, and some brass hardware store fittings. Make sure you bleed the air out of the hose/gauge too. A lot of oil pressure Test gauges ya buy are 0-150psi. That's a wide range if your oil psi is 10! Gauges read more accurately in the center 1/2 of their range. Change the filter and oil & repeat and see what happens. It's possible the filter is plugged....but that's wishful thinking. Have you checked compression too? Often the center cylinders burn a valve... I have one which is bout 75psi. It may go up off idle, IDK.
The oil pressure is low on my H3 3.7L engine too. About 10psi at warm idle ..hot day, in gear, foot on brake. I didn't have the line bled well enough in the pic below. It's betweeen 10-11. Off idle and at road speeds the PSI is ~25. Not the greatest, but seriously it's been like that for tens of thousands of miles, so I quit sweating it. Certainly not what I'd like it to be. But guess, what, the sun continues to rise in the east each day and driving my H3 you'd never know it. Technically speaking the oil pressure it's still within the minimum limit for most engines. I been putting 1 qt of 10-40 in mine each change to bring up the oil pressure a bit and the light and buzzer never go off. So you could try that.
There are oil squirters at the top of each connecting rod which technically squirt engine oil to the underside of the pistons. Presumably to cool them off so they can run a 10.3:1 compression ratio (10.1:1 early I-5's) ..which is pretty high for normal pump gas. Therefore 'techniclly speaking' running real thick oil where it doesn't squirt out, might overheat a cylinder and cause detonation or possibly burn a valve. But in practice some guys on this site have run higher weight oil seemingly ok. I just fudge a little on mine with a qt of 10-40. And like I say, it keeps the oil light from ever coming on.
The other thing you could do would be to pull it and install new bearings, oil pump and timing chain tensioners in it, a real 'quickie' re-do. Or maybe hone and put in new rings. The iron sleeves are high-nickle and often don't need to be bored. The you're likely to get many more 10's of thousands miles. Any used engine will probably suffer similar issues. These engines are well-built & can go 300k+ miles if not for the weaknesses listed herein. Best of luck.
