Actually, there are much better choices out there. Evinrude outboard gear lube for example is able to mix with up to 25% water and still provide adequate lubrication should your gearcase develop a leak. That is a true benefit in my book. Other engine manufacturer oils may carry that same benefit. Something you do not get with a generic oil
Second. Whether or not the engine has ever been run the water pump impeller is now 35 years old. Should be changed prior to running any length of time. I would not trust it. A 35 year old impeller will break the blades off in a heartbeat whether or not is has been run. If this happens your new engine just turned into scrap metal.
You will need to break the engine in properly. Your first tank of gas/oil should be mixed at a ratio of 24:1 instead of 50:1. Run the engine easy for the first tank.
Important. The vintage engine you have may a sticker over on the right side of the engine near the idle adjustment knob. This sticker will say 100:1. If ift does disregard that sticker. Am thinking this was for 1985 but check anyway. A dealer bulletin was sent out back then for dealers to notify customers to run on 50:1 and not 100:1. Those stickers were to have been removed but not many were.