i had the same probs with my 89 xt tenere on my ride to mongolia. if you look at the air box there is a drainage hole on it for the oil to seep out on the left which is probably how you noticed it. so it is probably supposed to leek abit, not sure why but maybe its to keep the air filter oiled to catch the dust. i have always had that problem but never caused any problem with the bike. oil got low every now and then but not by much. mine did leak quite bad when i first bought the bike but that was due to me over filling it, the best way to top the oil up is a mentioned as before, top up a bit, let her run for a bit then top her up some more. i never bothered with the dipstick as the tank is only a top up tank, as long as your tank is half full you good to go, any higher ie 3/4 full, you start getting alot of oil in your air intakes and she starts to smoke.
from what i found all xt's leak abit, i rebuilt mine from scratch for my ride, john, one of the other lads on the trip had his built for him and both of them leaked in the same places a little, its just the way it is, usualy from the alternator cover by the bottom dowl, the air box and from the the bottom of the crank case breather, as long as you not dramaticaly loosing oil in your oil tank, dont worry about it, they usualy block themselves up. xt are very hard to kill. great choice in bike and fantastic fuel concumption.
also. dont put semi oil in an xt cause it makes you clutch slip, only use mineral oil.
look at the punishment mine took. up dating the site soon

www.horizonsunlimited.com/tstories/patrick