XR200 bogging..plus other maintenance ?'s

my friend just bought a 1996 xr200. tonight when we were riding the bike started bogging, even if you revved it to the moon and dropped the clutch. I put the gas on reserve and it ran fine, then he tried it in the "on" position and it ran ok for about an hour then it continued bogging (he has lots of gas :).
we were looking in his tank before and we seen somethign that looked like a rock, i took the tank off and it almost looked like tar, when you shook it it stretched right out, then when the gas settled the particles came back together forming a clump the size of a nickel. Could this be causing the bike to bog (dirt in gas?). Also, what type of tranny oil should he use? is the drain bolt that big nut on the lower left side of the case below the gear shifter? (if so, what a stupid place for it, and the nut is at least 23 mil.) How much oil does it take? Im afriad the owner before him has never even changed the tranny oil.. :ohmy: sorry for all the questions, but ive never worked on a thumper, and he has no manual to work with.

P.S. a little tech question..does the tranny oil lubricate the top and bottom end on a 4 stroke such as an xr? but it doesnt actually burn oil..so a 4 stroke wouldnt smoke at all right?

thanks for the help bogde, that makes sense because the guy he bought the bike off didnt ride it for awhile. ya we cleaned the filter, I was amazed how quickly it got dirtied again, its so concealed, and well, its an XR! ;)