Been using oil with wrong viscosity for 10 years - a burnt valve taught me

I was rebuilding a Cummins ISX and found a burnt valve in cylinder 4. The guy I was helping asked what oil I ran and I told him 15W-40 like always. He pointed out the engine had over 800k miles and showed me how the bearing clearances had opened up over time. Switched to 20W-50 on his advice and the oil pressure came up 12 psi at operating temp. Any of you guys adjust oil weight for high mileage engines or just stick with the book spec?

2 comments

2 Comments

williamschmidt1d ago

You really saw a 12 psi bump just from going to 20W-50? That's a solid gain for an older engine. I've always wondered if running a heavier weight in a high-miler helps the bottom end live longer, or if it's just a band-aid for worn clearances.

jason_kelly81d ago

My 351W had 140k on it when I switched to 20W-50 and saw the pressure go from 38 psi at hot idle to 50 psi. @williamschmidt I always figured it's a band-aid for worn clearances, but does it actually help the bearings live longer or just mask the problem until something else fails? I've heard guys say the thicker oil doesn't flow as well on cold starts and that can cause more wear over time. What's your take on the tradeoff between hot pressure and cold start protection on a high-miler?