the old ford/ international navistar engiens are built to last if you take care of them. 65k miles is nthing for that motor. keep a constant eye on oil level and pressure. i had an 89 lock up because the oil sending unit failed so i would check the oil once a week and constantly monitor the oil pressure. stock they come with 190 hp. if yours has an after market turbo it will probly get 200, fuel milage will increse with a turbo too. people dont own these trucks for speed, but for their durability. Ford IDI motors are notoriously cold sensitive, keep your glow plugs working and plug it in in the winter or you will fry a starter. can also be ran entirely off of filtered waste vegitable oil with no problems. all in all a great motor. just keep it serviced and you can get 400k miles out of it.
P.S. if you want it to black smoke, you can manualy crank the injector pump up by removing the small cover on the left side (standing infront of the grill looking to your cab) and inserting a 5/16 allan wrench in it and giving it a quarter turn. HP and throtle response goes up too.
Seceded, DON'T run it off veg oil if you want long trouble free service from the fuel system.
Ford IDI motors are notoriously cold sensitive, keep your glow plugs working and plug it in in the winter or you will fry a starter.
Starters fry due excessive cranking because of a poorly maintained fuel and starting system. Mine and most others will start within 3 seconds without ever using the block heater at less than 10 degrees. Most everyone here who ran their pickups for an extended period without a turbo, then installed one agree, a turbo doesn't increase fuel mileage at all. I put 30K on mine, then installed a turbo kit, now have 245,000 fuel mileage stayed the same. There were two versions of these engines, a high and low altitude model, the high altitude version was 165 HP.
If your grandfather hasn't been using a suplemental coolant additive to prevent cylinder wall erosion it would be wise to start using it.
I'm not an auto trans guy, but Chuckster57 is knowledgable and has had great service out of his. His is the same as yours and he'd no doubt be happy to advise you as to oil change interval, whether you need an additional trans cooler and which one.
Heavy truck and diesel mechanic (thankfully retired after 30+ yrs)
'91 F-250 SC 4X4, 5spd. ATS Turbo, 3:55 diff.