I just recently came cross this mentioned in the Service manual of 92-93 Fleetwood FWD. "10W-40 is specifically NOT recommonded".

I believe that 10W-30 is what is listed in the owner's manual for the motor oil. And just about everywhere you would get this multi-grade oil for the oil change.

My question is this, Engines w/ higher mileage can use higher grade oil for the simple reason that engine parts are not as closely positioned as they were when they are new. Therefore, higher grade oil like 10W-40, or -50 will prevent leakage. Why is 10W-40 specifically NOT recommanded?

I think that is a hold over from the '80's when 10W40 was known to sludge up engines terribly. Todays 10W40 is probably OK to use.

Haha wow, I'm using Rotella 15W40!

But that's changing this weekend!

Rotella is excellent...no need to go away from it. Plus, it's not 10W-40.