Originally Posted By: fpracha
Originally Posted By: Jim Allen
I'll bet 90 percent of the people that owned and still own them use 15W40, though. In the late '80s/early '90s, the 15W40 HDEO was a better oil than the SF/CC stuff recommended by the factory. I think Rotella was at least SG/CE by them. The first time I really looked hard at a diesel API roundel and investigated what it meant was CF ('93 IIRC). Looking back at my maintenance records, I used a Pennzoil 10W30 from '87-ish to about '92 and then switched to 15W40 Rotella for a number of years until I found a regular supply of 10W30 RT.
So were there any changes in fuel economy when using the 15W40 versus the 10W30, considering the many times back and forth from each grade and also long enough runs on each grade ?
If no differences, would you attribute this to the fact that above 2200 RPM with the 15W40, the filter was in bypass hence allowing more oil flow than via the filter element ?
Good question but I don't have an accurate answer. During that period, the truck was primarily a tow vehicle. In snapshots since then, I would have to say that I can't see a difference. The difference I do see is in cold starts when the truck isn't plugged in (normally I plug the truck in). In my experience, the bigger the engine, the harder it is to measure viscosity related mpg changes. Also, because the engine has an oil to water oil cooler, the oil gets up to temp rapidly, so it might be an engine that would show viscosity-related mpg loss less than others (the warmup phase is where viscosity related parasitical losses are most extreme, so short-hopped engines will suffer more and long run engines less.)
The oil pressure deal remains a mystery to be solved. Anything I say now is just speculation until and unless I install a gauge again and do some investigating. During the overhaul, I changed one end of the oil cooler (like an idiot, during the overhaul I dropped it... it's cast aluminum... it broke and I was lucky to find a NOS replacement for about $200). That part has the filter mount and oil pressure relief valve.
Interestingly, when I tore the engine down, the lower end was perfect and perfectly clean. I had access to an engineer, bearing specialist, at Federal Mogul and had him "read" the bearings. While there was some evidence of cavitation, he pronounced I should reuse the bearings. I didn't, but could have. He suspected the cavitation was due to my engine having an aftermarket turbocharger on it (it was built naturally aspirated) for most of it's life. Nothing needed on the crankshaft or oil pump either. Small end rod bushings miked good. Cylinders had no more than about 0.001" taper on the worst one, so I didn't bore either. The valves and heads were an awful mess. Too much High EGT towing, dyno runs, etc.
