Two things I would to to visually inspect the oil. Is it liquid at room temp, and what color is the oil. Brand new oil is yellow or golden brown. Nasty used oil is dark brown or black. The oil is use ranges from golden to fairly dark brown. You can also put a small sample of it in a jar to see how clear it is. The more clear it is the better the oil. I would also suggest putting a jar of the oil in the frig and see what it does. That is what will be in the tank in the winter months.

As far as donut shops it probably depends on the donut shop. I would talk with the cooks. They will know what kind of oil they use. You want Canola or Soy. Stay away from the hydrogenated and partially hydrogenated oils. They gel at wamr temps and you don't want to be trying to pump solid oil in the winter. Avoid any place that goes through large volumes in short time periods. They tend to use crappy oil and it's usually really dirty.

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