Tallow contains animal oil (which can be extracted by simply pressing it). In fact, McDonalds for years cooked their fries in 97% beef tallow. Of the other 5 ingredients listed on a can of Proofide, 3 are either animal or vegetable oils. Proofide is primarily an oil based dressing with a bit of wax in it. Neatsfoot is also an animal oil which has been used successfully for hundreds of years to protect leather. Neither neatsfoot nor Proofide will ruin your saddle if you don't overuse them. Used in similar quantities, neatsfoot will soften a saddle a bit more than Proofide will but only a bit.

Brooks warns you not to use anything but Proofide because they can get $8 a can for it.