Debate: synthetic oil changed my engine vs. the old timers say stay with conventional

I run a 2001 Cummins 5.9 in my daily hauler. Switched from Rotella conventional to full synthetic at 300k miles. After 50k miles, my oil analysis showed way lower wear metals. But my buddy who's been wrenching since the 80s says synthetics cause seals to leak on high mileage engines and to stick with what's proven. He showed me a 7.3 Powerstroke that started dripping after switching. So which is it? The lab numbers or the 30 years of experience? Has anyone here seen a real difference in engine life from making the switch?

3 comments

3 Comments

veram991mo ago

Made the switch on my old 5.3 Chevy at 220k and saw the same thing - oil analysis looked way better after. The seal thing is real on some engines if they've been running conventional their whole life, but it's not a guarantee. I'd trust the lab numbers over any mechanic's gut feeling.

holly_henderson861mo ago

220k miles on conventional is a solid data point. My buddy's 4.3 Vortec had 180k when he switched to synthetic, his oil analysis showed iron levels dropped by half in two changes. The seal thing can happen but usually on engines with over 250k that have never seen synthetic and are already leaking from dry rotted seals. Lab numbers don't lie, that's the real story.

ross.felix1mo ago

@veram99 nailed it with the trust the lab numbers part. I've seen that same trend in the 5.3 and the 4.3 Vortec, it's pretty consistent once you hit the ~200k mark. If you're worried about seals, just do a high mileage synthetic blend first and see how it goes, that's what I've done on a couple higher mile motors and it worked fine.