The oil for a Generac generator is either SAE 30, 10W-30 or 5W-30, depending on the temperature. It is important to change the oil after the initial eight hours of operation, and then every 200 hours following the first oil change.
If the temperature is above 40 degrees Fahrenheit, use the SAE 30 in a Generac generator. If the temperature is cooler than that, all the way down to 10 F, it is necessary to use the 10W-30 oil in the generator. In sub-freezing temperatures below 10 F, use a synthetic 5W-30. It is important to use the generator outdoors and never start it indoors, even if the temperature is cold.