The recommended oil is determined by the pump manufacturer (usually different from the engine), but most of the "consumer grade" pressure washers say you never need to change the pump oil. Maybe they figure something else will break before it needs new pump oil?
The little jugs of pump oil at home improvement stores are usually SAE 30W (non-detergent) crankcase oil. 75W90 gear oil seems like a better choice; I personally use synthetic marine lower unit gear lube in my pressure washer pump. It's a little more expensive, but I think the formulation gives better moisture resistance. Besides, the pump takes a fairly small amount, and I don't change it very often at all.
Bottom line- you almost certainly want non-detergent oil, around 30W crankcase/80W gear (since they use different scales, these are just about the same viscosity). Synthetics seem to be higher quality, and using marine grade lube gives me peace of mind.
The pump oil matrix here is illustrative- note the many substitutions allowed. The specific oil you choose probably doesn't make much difference, especially compared to never changing it.
Testimonial: "Thank you, youre right about the marine grade oil I have some and I will try that, danmanthepickleman."
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