Re: 73 Merc 500 lower unit gear fluid
Any "marine gear oil for mechanical shift motors" is what you want to use. I recommend you get the gear oil pump as well. In your case, you might order two qts, so you have enough to change gear oil twice. It is avail at Walmart, Kmart and other department stores. You got any of those?
automotive gear oil is a last chance substitute. You do have that stuff, right?