Here's a link to your Simpson manual:
No oil is required for the pump.
10W-30 is normally specified for a Honda GCV160-GCV190 engine. You drain the oil by removing the oil cap/dipstick and then tilting the machine on it's side so that the oil can drain out if the oil fill into a container. (Drain when warm so it flows easily).
The capacity of the engine is ~18.6 oz. Refill amount is ~12.0-13.5 oz. (either engine).
When refilling, add oil and check on dipstick often. To check, press the dipstick into the oil fill, but DO NOT SCREW IT DOWN. Just insert the dipstick in until it contacts the top of the oil fill. The correct reading is between the lower and upper limits marked on the dipstick. (You may have to look carefully to see the marks).