Ayuh,..... That Guy, is talkin' out his butt,......
The differences 'tween automotive gear oil, 'n marine gear oil is the pretty colored dye they put in the marine oil,.....
That's why We have oil "Spec"s,.....
I believe it is a little more than just a different coloured dye. The gears in an I/O are very small compared to the gears in a car/truck, but they have to handle the same same power all the same, so perhaps the additive packages are a little different.
I know my uncle used to listen to his 'expert' friend who told him that the oils were the same. Every summer after he changed the oil, using automotive gear oil, the upper gearcase would blow. He would get it rebuilt, and it would survive the season. I convinced him to use OMC Hi-Vis in the fall oil change, He never lost another gearcase. Coincidence? Maybe, maybe not