This may sound like a smart alex comment but isn''t intended to be.....are you positive the stain is diesel fuel
? Teak veneer is quite thin...if others have attempted to remove the stain as well as yourself, you may actually be seeing an underlying ply of wood. (i.e. can you still see the teak grain or has it disappeared?)
My experience has been that it doesn''t take much to "go through" the thin layer of teak... Just a thought.
I''ve had good luck with TSP in cleaning up teak that has darkened over the years because of old teak oil. A strong solution may remove some of the softer grain in the teak if you brush it to much or use a stiff bristle brush, but the TSP pulls out the oil.