wood care/cleaning

the bass guitar than i own is a 1988 warwick streamer stage1. it has a flamed maple body with a natural oil finish. ive owned it for about 10yrs and i waxed it regularly for the first 5/6yrs whilst it was gigged. for the last few years, however, i have not. the area where i rest my thumb(about 1x1cm) has lost its colour, and dirt has built up in the grain where my wrist touches the body. does anyone have experience of this sort of finish, and if so can you recommend a way to clean the grain without making matters worse? at the moment the guitar still looks great, but i dont want to just wax over the dirt and i cant not wax it as the problem would get worse!