Originally Posted by bsms
This is more a grease than an oil, but I really like how it works: Dover Saddlery | Passier Lederbalsam Leather Care.
I've used it on saddles and boots. It seems to take out small scratches and doesn't leave it feeling oily. It doesn't soften it as much as neatsfoot oil, but it worked great on the store display Bates saddle I bought - dry and stiff when it arrived, but a couple of treatments and the leather is now great. I think it also has some wax in it, which helps keep the oil in the saddle longer.
I third Passier Lederbalsam! I have been using it for a decade or more; it's very good stuff. It's closer to the "goop" used by high quality leather tanners to condition and soften leather before it's made in to something, so it works very well on high quality leather items. I use it on all of my good saddles, bridlework, and leather halters. I put it on with my hands, to help warm the conditioner and flex the leather a little so it soaks in deeper. Adam's Horse Supply has the best price on Lederbalsam. Browsing Store - Passier Lederbalsam Leather Conditioner
If an item is very dry/stiff, I will first use Hydrophane Leather Dressing. It's a light-weight oil based product that helps waterproof the leather, won't damage stitching, and doesn't darken very much. Browsing Store - Hydrophane Leather Dressing - 17oz
After a few coats of Hydropane, I follow with the Passier Lederbalsam to soften and protect even more.