I use Yamaha regular, not HD, gear lube. I have a reason for this, although I am no scientist and may be mistaken. Yamaha offers two different types and I believe the last time I knew, so did Mercury. It makes no economic sense to do this unless there is a true reason. There are classifications GL4 and GL5. GL5 is more heavy duty (thus Yamaha calls theirs HD) but I am pretty sure it is HD because of certain additives, one of which is sulfur Sulfur is corrosive to yellow metals and OMC lower units are full of them. In my mind (a warped world indeed) maybe this is why I am seeing more and more OMC lower units with badly worn pinion and drive shaft bearings. So, I use Yamaha regular, which has a rating of GL4. I have never been told this in class but I suspect this is why their smaller engines use regular and 4.2 liter and up use HD. I know for sure that there are no yellow metals in their bigger lower units.
Long live American manufacturing!
