With 56,000 miles I changed the diff oil both front and rear. never changed untill now, went with Mobil 1 75w-90 synthetic. The gears looked good no metal. Thats the worst smell i have smelled. Had to jack the front up to get to one bolt.

Since new the engine has had Rotella dino oil untill this time. I put about 1200 miles on the last change of dino oil, then changed the oil filter and drain the oil. Put Rotella 5w-40 synthetic back in it.

The transmission fluid has never been changed. The trans shifts great. I dropped the pan changed the filter cleaned the pan and the reusable gasket. The fluid still had a red color to it. The pan was clean like new no metal. The magnet was a little dirty. So after i installed the pan and put 5 quarts of Vavoline ATF +4 back in it, took the trans cooler return line off of the transmission, connected a 5/8 I.D. hose to the trans line using a hose clamp. Dropped the other end of the hose in a clear bucket with a line marked on it for a gallon.
Cranked the 5.9 up and put in Netural then when the fluid in the bucket got close to the gallon mark, then shifted the trans into park and the fluid stops pumping. Pour in about 4 more quarts then shifted into netural and pumped out 4 more quarts. kept doing this untill i have poured in 16 quarts, which is how much fluid there is in the transmission. Also shifted into Reverse and all forward gears while the fluid was pumping out. The trans has now been flushed.

On a route to another state(125 miles) drive it alot, the overhead was reading about 2 MPG more than it had before all the fluid change, running the same route using the cruise control at about the same speed. i Haven't hand calculated it yet.