Howdy Guys,
Here's a couple of hints you can take to the barn and mull over. Many saddle makers I know will use Canola oil (Right! The same stuff you cook with.) as a finish oil on saddles. It pure vegetable oil, is compatible with most leather tannage and when dry leaves no residue.
Most think unless they have some thick layer of grease on their gear it ain't water proofed..not so...what matters is what gets in the leather fibers. Oil in the leather is what water proofs it not what is laying on the surface. Leather will never be totally water proofed...if that's what you want use a synthetic material..like they make poncho out of.
If you have soft leather items...gear bag, black powder bag, or perhaps an all Leather Day Pack you can use the same stuff used to water proof tents. Spray it on....once dry it leaves very little sent if any and no residue to get on cloths. Frankly if you're worried about cent learn to use the wind. Regardless of what you do we "stink" in the woods...and it isn't the leather or what you used to water proof it.
Commercial "grease" leather water proofing has a scent too...and will maintain the scent because it never really drys.
Dave Bulla has a good idea but it sounds like he isn't going to share his "secret ingredient". So, I wouldn't go there....it could take a life time of experimenting to get it right and could cause the lose of a lot of gear due to leather rot.
By the way, regarding leather rot...most folks put way way to much oil, leather dressing, or goop on their leather...most commercial or home made leather dressing will cause the leather fibers to break down if used to heavily and cause a shortening of the life of the leather items.. A little goes a long way....I suggest whatever you apply you apply with your finger tips...never use a rag.....massage it in with your fingers and you stand less chance of over oiling. A rag will put way to much oil on the leather.
Just my two cents from 25 years of being in the leather business.
Art