Will it make you sick if you use the oil in a deep fryer if it wasn't refrigerated overnight?

No. Why would it? You don't generally refrigerate deep fryer oil anyway so you will be fine.

By the time you get the oil up to frying temperature, any bacteria in it will have been killed by the heat. The only risk you run from not refrigerating deep fryer oil is that it will go rancid more quickly. It won't hurt you, it just won't taste very good.

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Originally Posted by brao28

Will it make you sick if you use the oil in a deep fryer if it wasn't refrigerated overnight?

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Generally no, but you should really do a quick taste/smell taste, if it has an unpleasant smell or bitter taste, it's probably rancid. No oil should be kept near a heat source, oils can be affected by light and heat, they should be stored in a cool dark place. The basic rule is oils with a higher sat fat content, i.e. lard, plam, coconut oil are fairly stable, they can stay unrerigerated for longer periods. Oils with high polyunsaturated or monosaturated fats such as safflower, sunflower are more delicate and should either be refrigerated, but oils like olive or canola will get cloudy. I sometimes keep peanut oil in the fryer overnight, but with any oil if your going to reuse it and it still has good color and smell I'd just keep in in a cool, dark place in a jar. Cleaning out your fryer after use will cut down on bacterial growth and the smell from the previous foods.;)