Tired of hearing everyone rave about synthetic oil for older diesels

I was up near Grand Rapids last winter working on a 1994 7.3L Powerstroke that had been running Rotella 15w-40 since new. The owner wanted me to switch it over to the new synthetic stuff because his buddy said it would help cold starts. After the change, it started weeping oil from the rear main seal within two weeks something it never did before with conventional. I ended up draining the synthetic and putting the old Rotella back in, and the leak stopped completely. Has anyone else seen seals get upset by switching oil types on a high mileage engine?

3 comments

3 Comments

wren6381mo ago

My buddy Rick had a 2001 Cummins with 280k on it, switched to synthetic one time and his front main started dripping within a month. He drained it, went back to conventional, and it sealed right up again. Some of these old diesels just don't like change I guess.

the_ruby1mo ago

So your buddy's truck just decided synthetic oil was the enemy after 280k miles of getting used to the cheap stuff? Sounds like that engine had a good thing going with all that sludge holding everything together. Guess some machines just need their grime.

hugoj121mo ago

Did you put synthetic in a motor that had decades of conventional sludge built up? That stuff cleans out gunk that was probably plugging your rear main seal. For high mileage engines, I stick with what was in it from day one - switching just asks for leaks.