start your engine from cold (whatever temp you happen to be standing in) 30w
oil. Listen to the sounds you engine makes. notice the difference in sounds
coming from the engine over the next 30 to 60 seconds. shut down your engine
and restart, listening to the sounds.

The sounds you hear first that went away in a minute or so is the sounds made
by engine bearings lacking in pressure oil

Now, do the same with 10w-40 and listen again.

end of report.

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

Exactly why I wonder if all of the "automobile" advice applies to marine
outboards. This is oil made for the auto industry where the "starting" range
is
0f or below to a normal operating temp of over 210f (what most auto
thermostats
run).
That is an operational range of 200 degrees or so.
Outboards seldom start below 70f and it is closer to 80 most of the time
here.
The thermostat is 120f.