Lexol...what he said. I also use Cetophyl, or the Wallyworld equivilent thereof. You can find it in "Health & Beauty", its used for really dry skin, like psoriasis (sp?). Stuff works wonders on human skin, so I thought, "Why not on dry, uncracked leather?"
The "raw" unterated 1/4 in. oak sole scraps I tried it on softened up, pliable. Cetophyl also darkend the leather to the "old school baseball glove brown" w/out any dies or other preperations.
I applied it to my very well made sheath that came w/ my Blackjack Halo 14 w/ fingergrooved moss green handles. Then I applied LEXOL in 5 coats, and 3 coats of Mink Oil.
Result?? The sheath looks as "old school chestnut brown" as possible, and has not deformed, rotted or drawn any water. The knife is A2, and I use EEZOX on the blade, and all the exposed tang and metal furnishings.
After over a year in everyday temps, humidity, storage IN THE LEATHER SHEATH, No leather rot, no shrinkage, stitch rot or other common problems.
Knife has been checked and EEZOX reapplied 3 times, because I am neurotic and picky. NO RUST, very small patina is easily polished off w/ Simichrome.
Lexol is a great product. If I could soak my black leather motorcycle jacket in a sealed tub o' that stuff, I would!