I’ve never worked in a restaurant and don’t know how long they keep their oil for. However, I used to engineer commercial biodiesel plants, and am very familiar

I’ve never worked in a restaurant and don’t know how long they keep their oil for. However, I used to engineer commercial biodiesel plants, and am very familiar with exactly how rancid, oxidized, and contaminated with solids used oil from restaurants often is- I’ve seen some truly nasty stuff and don’t eat fried food from restaurants. “food coming out of it looks and tastes and smells bad” is exactly right.