Oil changed with Mobil1 high mileage synthetic oil for most of its life. Don't let the mileage scare you. Oil changes have been regular. I use boesh oil filters. Fram air filters. Ive never ran "engine cleaners" or seafoam crap through it. Its drank exxon gas all of its life, not mom and pop gas. compression numbers are all acceptable at about 155 each cylinder. Oil is not leaking past the rings.. The spark plugs are dry and not oil soaked as one with an internal oil leak would.
To give you a picture on how this thing was driven, the first clutch was changed at 330,000 miles. Yes. The trans hasn't had to push around 35 inch tires or anything like that. It hasn't been dogged out or raced.
If you're that worried about it, get it rebuilt and stroked.
Obviously its lasted this long because we take care of our vehicles. Engines don't just last this long just because.
If you've got any questions, feel free to message me and I'll get back to you ASAP.