I have a Fender Stratocaster with a rosewood fretboard. The wood is fairly porous, and over time I have had something like dirt, dust, or possibly wax build up in the pores of the wood grain. The last time I restrung the guitar, I attempted to clean the fretboard with Kaiser Lem-Oil, and while it looked great while it was wet, as soon as the oil dried it looked like it had possibly even made the build-up in the pores even worse.
It is a little hard to see in pictures, but if you look at the following image you can see it pretty clearly in the higher frets on the right-hand side. Is there a safe way I can clean this and get the wood looking clean, smooth, and dark?