George...I am no expert, but Ive had good success with high quality artists grade acrylics on leather, but oils are a bit more p.c. ... I do think oils would be best, but will take longer to dry, smell worse if you use thinners and linseeds etc...and be much more of a pain to clean up after...
they both cost about the same and can easily run ten, twelve, or even fifteen bucks a tube for the basic crimsons, cobalts, Ivorys and "noir" blacks...my daughter gave me a few hundred dollars worth of her "left over" paints when she graduated her art and design classes at college, and Im still using those...I dont know if the cheaper "craft " grade little bottles of paint would work, but as watered as that stuff is I doubt it and wouldnt try them myself...but good quality air brush paints may be a cheaper alternative?
Good paint sticks like crazy to leather it seems, as many old bomber jackets seem to attest to...I have generally antiqued my painted creations back some after I paint them,, but never really technically seal them.
TCA