My 98 f150 4x2 nascar edition just turned 50k miles the other day. I'm pretty nuts about maintenance. I had the day off today and decided to put new brake pads on it, repack the front bearings, and change the oil in the diff. I've been waiting to put my internet ordered Royal Purple diff oil to the test. I was pretty shocked to see the condition of the rear end oil at 50k. It was black, and mucky looking. The previous owner towed his boat, and it looks like the diff was overheated and it was just generally a mess. Ford says the rear end is a 100k change? I would say 35-40k at most. Once a year if you do any towing. Don't forget about that diff. I went with the royal purple stuff because it got it on ebay pretty cheap, and it already has the friction modifier in it so I don't need to buy a seperate bottle at ford.