The only thing I can suggest is to make comparison with the different manufacturers of oils. Other than that, looks like dealers might have that area cornered. Seems like on some of the 'old units', you were able to use a 90wt gear lube but not in todays specifications applications.
Check your owners manual for the specs and use ONLY that spec or you will probably void any warranty you have left. For the price at the dealer, anything less is just not worth the risk. My opinion.
I know where your coming from Dave. Sometimes it takes a drive to get to a dealer and have to pay their price. Dealer hours are not convenient as well. Would be nice to find it on the shelves in auto stores and even Wally World or KMart. Like I said, its a cornered market.
To answer your question, I get the manufacturers brand for my 150hp Evinrude 2-stroke, high performance synthetic, at the local shop. But for my hunting mud motor lower unit, any high performance 80-90wt synthetic I get from Autozone has worked fine.