I have compiled this step by step guide on how to change a boat's stern drive gear oil.
I have also encountered some frustration in finding the proper tools that fit. I guess, so much for standardization! Hopefully this procedure will help you avoid those frustrations.
This procedure is based on a Volvo Penta SX stern drive.
I bought a gear lube pump that says "also fits a Volvo". So I thought, that is a good start! To better complement that, although a little pricy, I also bought a Volvo Penta gear lube. Halfway through my maintenance, I noticed that the gear lube pump cap was smaller in diameter than the oil container. In fact when I went around the stores, all the gear lube pumps were that way! It took me the rest of the afternoon to find a general-purpose fluid pump that has the same diameter as the Volvo Penta oil container. I rigged it by connecting the fill tube of the older pump (2) to the fill tube of this general-purpose pump (1) (see Pictures below).
Luckily it fit snugly!
So much for that! Here is the procedure to changing the Gear Oil.