Switched from synthetic to conventional oil in my 7.3 and here's what happened

I used to swear by full synthetic oil for everything. Ran Rotella T6 in my 1999 F-350 for like 4 years straight. Then my buddy Dave, who's been turning wrenches since before I was born, told me I was wasting money on an old high-mileage engine that leaks anyway. He said the 7.3 Powerstroke was designed for conventional 15w-40 and the thicker base helps keep the injectors quiet longer. So I swapped to Shell Rotella T4 conventional about 6 months ago. Truck actually runs smoother now and the oil leak I had on the rear main seal slowed way down. Has anyone else had better luck going back to conventional on these old IDI engines?

3 comments

3 Comments

corablack24d ago

Dave's been around 7.3s since they were new and told me the same thing about thick oil quieting the injectors.

elizabethmason24d ago

corablack's Dave knows what's up lol. I ran 5w40 Rotella T6 in my old 7.3 for years after a buddy at the diesel shop told me the same thing. The injector clatter is a lot more tolerable with the thicker oil, especially when it's cold out. Just make sure you're not in a super cold climate though, cause that thick stuff can be a bear to crank in like negative temps. I'd also swap the oil filter for a Motorcraft one when you switch weights, helps keep the pressure right. It's not gonna fix a tired injector completely but it definitely takes the edge off that rattle.

jasonallen24d ago

Honestly, I read a writeup on a Powerstroke forum a while back where a guy tested different oils and showed pressure readings, and it backed up everything @elizabethmason is saying here. The Rotella T6 5w40 really does help the 7.3 injectors stay quieter because of the higher viscosity when hot. But yeah, that cold start thing is real, I remember my buddy in Montana had to switch back to 15w40 in the winter cause his truck wouldn't fire below zero. The Motorcraft filter tip is solid too, those aftermarket ones sometimes have a weaker bypass valve that drops oil pressure at idle. It's not a magic fix but definitely a solid bandaid for an old rattly motor.