I've used the USP grade from the drug store (over 50 yrs) as well as from Sig (over 45 yrs) and I never saw any difference in it. Back 40 yrs ago I made up some homebrew using the drug store USP and it ran OK but was no nitro. I've never had any trouble with the engines from any of it.

Last year I noticed a 8oz bottle of USP castor that had been on the back of the work bench for for some 35 years. It had been opened and a small amount used back then. It wasn't rancid or anything. I used it in the fuel. It's all gone now.