I know you feel like an engine with only 11k on the block after 20 years is a real find, but honestly it might not be.
Oil is only one small item. Hopefully the engine was ran at the most ideal times so that the oil kept internal rust buildup in check. For example if he put on 11k the first year then sat in a barn for 19 years you'll have problems. Doesn't matter what type of oil you'll use, you'll have problems. On the other hand if he did a 100 mile run every few months for 20 years then you got a winner.
Also oil is one of the smallest of things in my mind right now. With so few miles I'm guessing maintenance fell to the wayside. Thinking you got original antifreeze, fuel filter, brake fluid, grease, and transmission fluid all in there from 1997.
I would run that unit very lightly in the beginning and keep a sharp eye out for puddles. Guessing that tranny could be a leaker, and maybe those brakes will lock up after some hard stopping/miles. You might have a great rig there, but I would baby it in the very beginning till the cobwebs are out of it.
Sidebar, that link to the oil made me laugh. Guessing he never rebuilt an engine in his life. Being book smart is not the same as being street smart.