If you are referring to high performance marine gear lube and NOT some generic 75W-90 gear oil not made for the marine environment, that should be fine for your conventional shift lower unit.
There are some really old dinosaur early 1970's Evinrude/Johnson units that use some wierd lower unit shifting mechanisms which required "Hypoid oil".
The non-marine gear lube does not have the additives to help protect the gearcase if a substantial but not fatal amount of water is in the gearcase while the motor is running. Stay away from that stuff for your boat.
Me, I don't plan on selectively buying synthetic marine lower unit lube. A good enough lower unit lube is all I need running my V4 90HP Johnson. That ends up being Quicksilver High Performance Gear lube.
If they end up having it roughly the same price, I'll get the synthetic. Otherwise it's like buying 91 octane gas when you only need 87 in your boat/vehicle.
Synthetic really only shines in high use & abuse environments or high horsepower because the gear lube gets quite hot in those engines.
