Used to hate synthetic oil in older Cummins engines, not anymore

Ngl I was always a Rotella 15W-40 guy for my 5.9L, figured the old girl didn't need anything fancy. Then my buddy with a '97 12-valve in Denver showed me his oil analysis after switching to a 5W-40 synthetic blend. His wear metals dropped by over 30% after 6 months with the same driving habits. Has anyone else seen real proof on their own trucks or am I just getting sold on hype?

3 comments

3 Comments

ninaowens1mo ago

Lol yeah man I was totally the same way, swore by the conventional Rotella in my old 7.3L and thought synthetic was a waste of money. But then I saw a video from some diesel shop in Texas where they ran a stroker 6BT on synthetic and the tear down after 200k miles looked like it was barely broken in. That changed my mind real quick. I switched to a full synthetic 5W-40 in my truck and noticed the cold starts are way easier in the winter, plus my oil pressure gauge doesnt jump around as much when I'm hauling. The wear metal drop your buddy saw is legit from what I've read, those long chain molecules just hold up better in the high heat cycles these old engines put out.

ray_martinez821mo ago

Texas shop confirmed it for me too, I switched and never looked back.

uma_williams1mo ago

I get what you're saying about that Texas shop video, but I still think synthetic is overhyped for most people. "Barely broken in" at 200k miles sounds great, but how many of us are actually keeping trucks that long? Most guys trade them in around 150k. I've seen plenty of 7.3Ls hit 300k on conventional Rotella with regular changes, no issues at all. The cold starts being easier are real, I'll give you that, but I think the oil pressure thing is more about the weight than the synthetic part. A good 5W-40 conventional would do the same thing. Plus synthetic costs almost double. For what I spend on oil changes over 5 years, that's real money I could put into fuel or tires. I'm not saying synthetic is bad, I'm saying the benefits don't add up for the average guy just hauling a trailer on weekends.