It is a rule of thumb that if you want to find a good roadside restaurant, you should look to see where the trucks stop. A more reliable but less accessible way to find a restaurant with quality food is to ask a used oil scavenger. We see how picky they are about their deep frying oil... If it is a light to medium brown and fairly free of gunk, the kitchen is trustworthy. If it is colored like coffee and full of chunks, you would want to avoid the restaurant... and avoid their oil!
Once upon a time I was fairly desperate for oil, and I took some from a gas station/ convenience store/ takeout counter. What a mistake... The oil was overused and had been overheated as well. It set for a long time while I tried to ignore it.
Processing it conventionally, even if successful, would have created a lot of waste. I opted to try an acid process that "restores" the oil before running it through the usual reaction to create biodiesel. More details are available at sites like journeytoforever.com.