Called BS on synthetic oil for 10 years until a teardown proved me wrong

I used to swear by conventional 15w40 in my old Cummins 5.9. Ran it over 300k miles with cheap Rotella T. Then I bought a truck from a guy who only used synthetic Rotella T6. Pulled the valve cover at 200k and the difference was night and day. No sludge, no varnish, just clean metal. I still think synthetics are overpriced but that visual convinced me it matters for long term. Anyone else change their mind after seeing something firsthand?

2 comments

2 Comments

jake_dixon1mo ago

Man that's the exact kind of thing that made me switch. Seeing is believing, right? I had a buddy who ran a 7.3 Powerstroke on conventional for 400k miles and it looked like a greasy mess inside when he pulled the heads. Then another guy with the same truck on synthetic and it was like looking at a brand new engine. The heat cycling thing makes sense to me now too, synthetic handles the hot cold swings way better. Still hurts to pay the extra cash but I figure it's cheap insurance compared to a rebuild.

caleb_gibson1mo ago

Hang on man, I gotta push back a little on that. I've seen plenty of old gas engines with over 200k on dinosaur oil that still looked pretty clean inside, way better than what your buddy saw. A lot of it comes down to how often you change the oil, not just what brand. If you're running 10,000 mile intervals on conventional, yeah, it's gonna look like a swamp. But swap it every 3,000 and keep it topped off, I think you'd be surprised how good it can look. Synthetic feels like overkill for a lot of daily drivers that never see a heavy load or extreme cold. I'd rather spend that extra cash on more frequent changes with the cheap stuff.