I've said it before, I'll say it again. Mink Oil.
The mutts nuts, you can get it from Le Prevo. I use it on all my leather and it is superb. As an example, I have a pair of issue assault boots and spent three months this winter stomping through mud in our woods claiming the windfall branches for my fire - they were scuffed, covered in pine needles and resign and coated with 3 or 4mm of mud.
I decided they must be cleaned and re-proofed. So I got an old nail brush and cleaned.
They didn't need re-oiling! Amazing stuff. Honest.