I owned a 350 xt in the late 80s. I treated it pretty like a bigger bike and wrecked the top end ( valves etc replaced under warranty).
After that I took it steady, used good oil, frequent changes and it lasted ubtil I sold it with about 30 000 klms.
What points I do remember include,
could be tricky to kickstart hot or cold,
not too bad off road but the hard starting could be annoying if you stalled in a muddy hole
with leaded petrol it would fly - I travelled from Leeds to the South of France in a couple of days for the Bol D`or ( about 1000klms)
Would I buy another? Probably not but that is because I have a KLR 250 and suits me well. If I needed something bigger I would go for a 600 XT as they are better catered for re spares and knowledge and probably do not weigh that much more.
I do not remember mpg or tyre usage etc but I do not remember it as been a problem.
All the best