When changing the oil in a pressure washer, use oil that is specifically formulated for the unit, rather than standard vehicle oil. The oil is designed to lubricate the water pump under high pressure.
The water pump oil needs to be changed at least once a year or after 50 hours of washing. Prior to changing the oil, clean the washer of any contaminants by removing the spray nozzle and allowing water to flow through the wand for at least two minutes. Empty the old oil into an appropriate container and pour in the new oil. Some pressure washer models include dipsticks to measure oil, while others simply use an entire bottle of the specially formulated oil.