Tallow is a fat, not an oil. It can be 'rendered down' to produce oil, of course, and I'd imagine that McD. used the renderings in their cuisine.
Unless you're going to cook your saddle you need to keep the oil to a minimum, and this is why any abnormal heat should be avoided when using 'Proofide'. The amount of oil that will penetrate your saddle, if you apply 'Proofide' correctly, will be negligible.
The frequent suggestion that Brooks only advise use of 'Proofide' because they make money out of it is absurd - a cynic might suggest that they would be better advised to encourage the use of neatsfoot, etc. (which they did, at one time), as this would probably significantly increase their sales of new saddles. A tin of 'Proofide', used sensibly, will last for very many years (unless you've got a shed-full to service, of course).
It's probably not too important whether you use 'Proofide' on your saddle, or not - it's the overall care and attention that's important. Obviously, quality of leather can vary tremendously (even within the same model) and some saddles will never be much good whatever you do with them (it's generally agreed that the overall quality of Brooks saddles deteriorated badly in the late '80's/early '90's, when they were being screwed by a parent company desperate to save money where-ever possible), but a good leather saddle should last a lifetime, given appropriate care.
If you want a saddle that is soft and/or waterproof there is a wide choice available - just don't waste your money on a leather one. Or on the neatsfoot, with which to 'treat' it.