Motor oil will be a disaster, don't do it.

I have had good luck w/ no treatment whatsoever on leather saddles. Stuff a plastic bag under it so the bottom doesn't get wet while you ride, then put the plastic bag on the top when you park it so it doesn't get rained on when your butt's not on it, and that will last many thousands of miles.

Proofide as such is not necessary. Any kind of beeswax for waterproofing at least won't hurt the saddle like motor oil will.