My point is Merc says to use 10w-30 OR 25W-40. (the only grades they sell)
30HD seems to be right in the middle.
Merc doesn't make 30HD and I suspect that has more to do with the
recomendation
than anything.

Merc doesn't make any oil, it merely repackages it (or more accurately, it has
it repackaged for them).

straight weight oils have not be recommended by major engine manufacturers (of
these kinds of engines) for a couple decades. multi grade oils were developed
to give better lubrication upon engine startup while still giving proper
lubrication at operating termps and loads.