Dave's right about the make up of Pure Neetsfoot oil, however Prime Neetsfoot oil has been around for along time & contains petroleum distillates. I don't use it for the same reason Dave doesn't, but I assume many do. It's still available, so obviously enough ppl use it without complaints to keep making it. So, I guess in a pinch the motor oil may not hurt. Also I have heard from ol' timers that olive oil does work well if used sparingly. Roger (Beesachoppa) has used it in the past, but I've never tried it. Neetsfoot oil is too cheap to chance a rancid leather seat later with olive oil.
As far as water repellent I've used Aussies, Pecards is better, but IMO Montana Pitch Blend works the best, although I haven't tried the Snow Seal, or Snowproof yet.
So, with that said here is my progression of finishes.
1. oil (neetsfoot, or lexol for natural)
2. dye
3. lexol
4. Leather balm with atom wax
5. Aussies, Pecards, or Montana Pitch Blend