I had a '94 XR250 for a year..... Ran really nice all types of roads/terrain.
I did change oil every 1000 miles, semi synthetic mineral oil.
As for air, oil or water cooled.... they are all designed to work as manufactured. Personally
, water cooled is added complication & risk of leaks
so I prefer air cooled.
I also run a couple of Kawasaki Z1000's an early 70's design... 11.5 sec @ 116mph - over 1/4 mile...... and
also ran it to York & back this summer, 650+ miles in a weekend, no probs
Any bike thats looked after & not treated like sh*te should be fine