So what would you say for my new to me 1987 F250 460 w/ 71,000 miles?

No intention of selling it anytime soon, and I know the guy I bought it from has done twice a year changes with Pennzoil.

 

I'd say that's roughly the highest I'd bother with going to full synthetic. But if you do ever sell it it will be for naught. Then again it could get totaled next year and end up being for nothing anyway (unless you buy it back and pull the engine).

And when I say keep if forever, keep in mind the transmission will probably go out before the engine anyway. Do you love the truck enough to put in a new trans when it takes a dump? (Or pull the engine and drop it in something else)