1998 E150 4.6 with 153k. Just bought it this week end. Previous owner regularly changed oil, M-1, every 5k. He either hauled a 400lb bike plus tools and other parts inside, or pulled an open trailer with two bikes on it accross the country several times a year. He finally changed the original plugs/wires several months ago (said mpg highway jumped from 17 to 19 @ 75-80).
Never an engine problem, receipts show tensioner belt pulley around the 70k mark. Trans was rebuilt 10k ago but it was shifting fine, problem was a cracked case. Shap said since it would run $800 just to fix that, he might as well spend a little more and have it rebuilt since it already had 140k. Off topic, they told him there was corrosion between the steel trans mount and aluminum case, built up enough to crack the case?!?!?!?!?! Personally I never heard of that before.
BTW the 250 mile drive home with it, cruise set on 65mph 99.8% of the way, light traffic, she gave me 21.75 mpg!