20 is probably OK if you don't push the engine hard but I agree with Chuck33079 that you will be better off running what the manual calls for.
When the oil is cold, 5W20 is as thick as straight 5 weight oil would be at that temperature. When the oil warms up, it's going to be like straight 20 weight. For winterizing, you only need to worry about the number before the "W". If you think you need a lighter oil for Winter, go to 0W30 - it will act like 0 weight oil when cold (for easier starts and faster pressure buildup) but still provide protection when the oil warms up.
BTW, changing the oil for Winter is usually done if the car is going into storage, If you are driving the car on a regular basis, there is no need to alter your change interval.
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