AR630 TSS High-Performance Pump Oil - Premium Lubricant for Hydraulic Systems, Machinery & Automotive Use - Ideal for Industrial, Marine & Heavy Equipment Applications

One of the best things about higher-end pressure washes is the fact that you can rebuild them. You'll want to do that maybe once a year, depending on use; in my case, it's once every few years.

This specifically is the oil you'll need to replace in your pump after doing a rebuild. I could give you my take on how to use it, but I think AR put it very well in the user manual, so ill quote that here. 

"Change oil after the first 50 hours of use. Use a SAE20 or 30 nondetergent oil. After that, change the oil every 200 hours, once a year or when it looks milky, more frequently if used in demanding environments. Changing oil is one of the most important and least expensive maintenance items, it will prolong the life of your pressure washer."

They don't give a specific amount you should put in, but there is a dipstick with the unit, so just fill slowly and check a lot, and you'll be fine.