jBrookshire, I know a guy in Wadesville Indiana who used to have a '91 with just a few miles on it. Mostly highway miles, although close to 50% of his mileage was with a Dutchman camper in tow.
Last I talked to him before he sold his truck he was sitting around 1,240,000 miles. Never had the head or oil pan off the original engine. He replaced the entire injection system around 600,000 miles. Amazingly the original turbo was still huffing just fine. He drove it another 8-10 months after that so he was close to 1. 3 million when he finally got a newer truck.
The truck has since gone to a farmer in Kentucky who bought it to tow his horses around. I don't have any updates on it at this time though.
He said it was starting to use a little oil at 1. 2+ million. . . 1 quart every 2,000 miles

He was trekking cross country with it regularly pulling trailers all the way up until he sold it. His job was delivering campers which he used his truck for from day one.
Vaughn