Re: 1978 omc sterndrive service
I have that same drive. I used to really like the Sierra "premium" gear oil. Sierra changed the oils they sell and they now call it "premium blend". I don't know what the difference is but I liked the old oil better. Don't have a good reason.
I think Sierra is still a good choice though. Real expensive. You can also get any old premium marine rated gear oil and it will work fine. Save some money if you like.
My advice to you is to replace all the gaskets on all the screws every time you change the oil. Use the nice nylon (medium hard plastic) ones. Use a new one everytime. And replace the rubber gasket on the top screw in the upper and the intermediate. It's like a rubber o ring. Tighten the crap out of all the screws when you have all the oil replaced. The screw has a bead on the underside of the shoulder and that seals into the nylon gasket. If you try to use the gasket a second time the bead can crosss the first seal and you'll get water in your drive. You probably will anyway, but you may as well eliminate as many possible intrusions as you can.
Also fill each drive from the bottom screw with a pump. I pump the upper and intermediate full until oil comes out the top screw. I pump the exact amount into the lower from the bottom. I think it's 38 ounces. Check a manual though. Someone may correct me, but I don't think it's wrong to pump oil from the bottom until it comes out the top and then put the top screw in an then remove the pump and replace the lower screw. Do the same for all three units.
Enjoy your old OMC while you can
