The bottom line is that motor oil is not suitable for pressure washer pumps. Because pumps do not have oil filters hence risk damage due to impurities in motor oils; always use non-detergent oils. And when it comes to replacing used lubricant that has become less viscous, do not overfill the pump. An oil leak is the worst problem you want to deal with, let alone damaged seals. Here you can find Pressure washer pump oil. Most importantly, stick to the manufacturer’s instructions on the user manual, which includes safe disposal of used oil to a recycling plant. You would rather be safe than sorry.