Responding to Albert de Boer's question, I have found Naphtha,
a commercial solvent, to work well for cleaning leather without
disturbing it otherwise.  I suppose it could dry out the leather if
used excessively, but rubbing it on the surface a bit to remove dirt
should be okay.

Naphtha is also sometimes effective to remove oil stains from wood,
and can remove the stains left by adhesive from masking and other kinds
of tape, in many cases (but not all) without disturbing the finish on
the wood.

Larry Broadmoore