I had a pair of these that I sold, I dilled and tapped a 1/4 npt hole in the bottom and put the plug in, this way you just remove the drain plug and refill from the upper plug,

Installing a drain plug is the best way to do this service. Next best way is to completely remove the pump every season then you can r&r the impeller housing, impeller, and the lube in the reservoir on a bench. Use drive lube in the reservoir. On 525/600/700 it is extremely important to consistently run a fresh impeller and housing for 100% water flow, not water pressure, to keep the headers as cool as possible for longest possible header service life.

suck it out with a turkey baster.........lol........if you don't install the drain plug

I use a good size hospital syringe, with some clear tubing attached to the end. I fish the tubing into the plug hole suck out as much fluid as I can. Then, switch the clear tubing to a fresh piece and fill syringe with gear lube and push in to fill. Kinda tricky, to fill with one hand and have plug ready to go in the other very quickly. Of note, when you pull the plug, its got like 2 threads, so very short. Have bucket or towels underneath as very thin gas laden gear oil will come flowing out. Its the fuel contamination in the pump fluid that happens as to why it must be changed. The gear lube integrity gets compromised... Good Luck!