leather treatment is a good idea.
I save all old oils for eventual soapmaking (I have an old soda bottle that any off-smelling oil goes into). But then again I don't make fancy soap, I make soap out of used fry oil and other discards. It takes me years to get enough to make a batch, but it also takes me years to use it. My mother in law taught me to make it using rancid lard, and I don't think it gets more disgusting than that. It turns out just fine though (as long as you strain out all the bits), no residual stink. This is cold process lye soap, probably more lye than really necessary but it's great for heavy cleaning.