Most pressure washers do well using 75W90 nondetergent oil. This is a very thick oil with a high viscosity. Each pressure washer manufacturer, though, is likely to specify the weight and type of oil needed in a given pressure washer's manual.
Nondetergent oil is recommended for pressure washers because it avoids creating the bubbles that occur with detergent oil. These bubbles can prevent proper lubrication and can combine with other fluids present in a pressure washer to cause the washer's pump to fail. While some pressure washers do not require regular oil changes, most manufacturers recommend changing the oil after every 50 hours of washing or once a year.