Like most mechanical machines, an Air Compressor needs oil to prevent the internal pump from seizing.
Bought brand new, the Air Compressor will have no oil, so as soon as you get it home you will need to put some in.
For all RYOBI Air Compressors with a wet sump, the specified oils are synthetic oil SAE-30. This type of oil is commonly used in small engines such as outdoor equipment and 4-stroke lawn mowers.
Only fill your RYOBI Air Compressor with synthetic oil SAE-30. This type of oil is commonly used in small engines such as outdoor power equipment and/or 4 Stroke Lawn Mowers.
It isn't recommended to use any variable temperature viscosity oils (for example 5w-30) as these oils thicken less in colder weather and may end up harming the pump and motor seals.
For optimum Air Compressor performance, it's recommended to replace the oil after the first 10 hours of the unit’s operation, and then every 100 hours following the first oil change. See: How to put oil in RYOBI Air Compressor