An old diesel mechanic told me to stop using synthetic oil in my 7.3 Powerstroke

Honestly, this guy named Mike at a shop in Phoenix told me that synthetics are too thin for high mileage IDI engines. I figured he was just stuck in his ways, but after 8 months, I started losing oil pressure on hot days. Swapped back to 15w-40 conventional like he said, and the pressure gauge stopped bouncing around. Has anyone else had a shift in oil type fix a weird problem like that?

2 comments

2 Comments

kelly.robin14d ago

Your pressure gauge "bouncing around" after switching oils sounds more like a coincidence than a real fix to me. These old engines have a lot of things that can fail or wear out, and blaming the oil type is usually a shot in the dark. Unless your engine was straight up knocking or smoking, the gauge moving a little on a hot day is probably just your sending unit dying or a bit of sludge finally plugging a passage. People get all worked up over oil weight when the real problem is a 30 year old engine with 200,000 miles on it. If it runs okay and doesn't leak, I would not overthink it.

kaib1814d ago

Oh man, @kelly.robin I gotta push back a little here. I know what you're saying about old engines and worn parts, but oil weight absolutely can make a difference on a high mileage motor. Going from a thin 5W30 to something like a 10W40 or 15W50 can totally bump up pressure readings by a few PSI at idle, especially when the engine is hot and clearances are loose. The gauge bouncing around might not be the sending unit dying, it could just be the oil getting thin when temps spike and the pump struggling to keep up. I've seen it myself on older Hondas and even some Chevy 350s where a heavier oil settled things right down. Yeah, sludge or a dying sender is possible, but dismissing oil type as a coincidence isn't really fair. If it runs fine and doesn't leak like you said, trying a different weight is a cheap and easy thing to test before you start swapping sensors.