I'll go against the grain and state that any GL-5 oil is good (Marine branded is not necessary) BUT use the weight specified by your manual. My current outboard's instructions say, verbatim: "You can use any brand of lubricant that has a grade designation of API GL5 SAE 80 or 90 weight."
Use the right weight. Use marine oil if you can get it; it may help and won't hurt. But don't be afraid to use anything that meets the spec.
Ill agree with you to a point as well!
I would say, first follow the manual for the manufacturer.
After decades of owning MerCruisers mostly salt water but now fresh I always used their gear lube, never an issue. I liked the "stickiness" of it, many years ago I tried Pennzoil marine gear lube since I now boat in fresh water not as big a deal, felt so light and non sticky I drained it a couple weeks later and put the merc back in. Here is an interesting test, get some merc gear lube on your hands and try to wash it off, you just cant, then try the Pennzoil and its no problem washing it off ... interesting.
Well fast forward to today! For the first time I own a Volvo Penta. Anyway, manual says to use Volvo Penta Gear Lube or Mobil 1 (formally Mobilube SCH) Delvac gear lube which is what I am about to put in with no reservations simply because Volvo says to choose only those two, its also ref GL5. Im kind of excited actually, never knew about Delvac gear lube, seems it is used in some high end vehicles(among Porsche enthusiasts) in addition to the heavy duty commercial applications which its designed for, also has the MT spec.