Quicksilver is Mercury's brand, it's typically decent quality. Just check the spec on the bottle against the spec in the motor manual. If the specs match then you're fine. You don't need specifically "marine" engine oil, if the specs match then you're good.
I've been using that gear oil in my 50HP Honda for years with success. I also use it in my 6HP Tohatsu, but not many hours on that motor yet. You're going to want a little pump for the gear oil. Gear oil is filled from the bottom-up, so you have to pump it into the drain hole. They make little plastic hand pumps that insert into the gear oil bottle for this. I've found the most convenient way to store the oil bottle and pump is to leave the pump in the bottle, use a little zip tie to hold the end of the hose up with the top of the pump so the oil doesn't siphon out, and drape an open zip-lock freezer bag over top as a dust cover to keep the dust and dirt out. Now you don't have to take the pump in and out of the bottle, and it stays clean.
