What type of oil is needed for a Honda GC160 Power Washer? Honda 2500 PSI x 2.3 GPM.

The type of oil recommended for that engine is SAE 10W-30 and the capacity is 0.61 quarts. This and more can be referenced at the website at the end of this solution. I was able to find the owners manual of your particular engine.

http://www.northerntool.com/downloads/manuals/60594.pdf?cm_re=Product%20Page-_-Manual-_-Side

It says 0.58L, but I seemed to get two fills out of a 1L bottle.

NOTE: normally, 2 types of oil is needed, one for the "Engine" as previously provided, and the other type is for the pressure washer 'Pump'. This in most cases is a Non Detergent 30 weight SAE. Non detergent blends will Not Foam up internally, that is a no no.