Just a wild guess here - was it run on gas from the wellhead? That gas might have carried some goo in that went past the rings and reacted with the motor oil, turning it to jelly. I think polymerization is the term.
Any chance it overheated? Does it have automatic shutdowns (oil pressure, water temp), and do they still work? Or were they bypassed?
What about maintenance? Did it get regular oil changes? Or did somebody try to get a zillion hours out of some synthetic motor oil blended with mystery oil and STP?
When I was young, my dad was showing me an engine that my older brother had destroyed by throwing a rod. He had taken the oil pan off and was scraping out the jellied motor oil with a putty knife - it wouldn't pour out at all. "You know, son, you can't just add a little STP and keep driving. Sooner or later you got to change the oil." I learned a lot from my older brother...