It's the only oil I have ever had loose all semblance of viscosity in under 300 miles - I won't EVER use it again. YMMV.
The OPR valve is in the front cover behind the oil filter housing, or in the filter housing itself. Can't remember for sure, but it is not a SIMPLE job to get to it in the car.Not rocket science either if you can work on a hoist and don't mind skinning a few knuckles. Only something like 4 bolts and 2 gaskets after you remove the oil cooler adapter (if so equipped)
GM SPECIFICALLY warns against using 10W-40 in most 3.8L/3800 V-6 engines.
I've used Pennzoil in GM, Ford and AMC engines for over 35 years and I've never experienced the problem you describe.
The more I think about this the more I'm convinced that the problem just may be the pressure relief valve hung open (or nearly so) by either a piece of debris or possibly the spring just gave up the ghost.
Before I dug into the engine any further I'd have the oil filter adaptor removed, pull out the PRV and the spring, then check the valve bore for any crap stuck in it. Also check the spring and the valve itself (a piece of debris stuck to the valve itself or a metal burr on the valve could make it hang in the bore).
If the pressure relief valve checks out OK, then you're either looking at a blocked pickup screen or another blockage on the suction side of the oil pump, a clearance problem on the pressure side (main, cam or rod bearings) that is leaking off all the oild pressure, or just a worn out oil pump (which is contained within the front cover).
Do not use 10W40 for extended drain. I'll agree with that - but I won't go for extended drain on 5W20 or 5W30 either. I'm running 10W40 on my 3800 and will continue to do so.
Hey, I've used it before too - but never since. I'd heard of the problem before, but when it got ME I sat up and took notice. Was it just a fluke? Quite possibly - but once bitten, twice shy. Quaker State and Penzoil are both off my "recommended" list, as are the low end of ANY manufacturer's product line. I've been a mechanic since the late sixties and I've NEVER had an engine develop camshaft problems or bearing problems - including rocker shaft wear, following my recommendation, while those following factory recommendations were busy with engine overhauls. I don't like engine overhauls well enough to inflict them on myself or my customers. Also never had advanced timing chain wear - even on the dreaded 2.6 Mitsubishi Hemi. My recommendation? a wide range multigrade changed oftener than "necessary". Four changes a year, minimum. 3000 miles max per change, unless that 3000 goes on in less than 6 weeks.And the only 5w rated oil I would use would be 5W40 or better (so you can see I'm not using much)
I'm suspicious, given the history, that the OPR is gummy. It's a VERY low mileage car that was likely undermaintained by my recommendation. Very unlikely the spring has failed.
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