No fight here. If I restored older engines, I'd be using 10W40 or 15W40 diesel oil too. Maybe even 20W50 !!
And you said most of your engines are used in 90° temps. They will be harder to pull start. But if given 15-30 seconds of idling to warm up they'll be fine.
Cold viscosity is not a moot point, it's one of many considerations. And yes, it's ranking is way on down the list, but that hardly makes it moot. As already pointed out, cranking will be harder. Pull cords may wear out faster. And on top of that the "40" won't really start to lubricate for several minutes. Luckily you'll be covered by the 10 in the 10W40.
Many people believe that just because the metal of the piston and cylinder get hot that they are "good to go". It's not the metal that has trouble warming to temp... it's the oil that needs to come up to temp. Oil most definitely has a preferred operating temp range, both for the ambient air temp AND the engine operating temp. A look at your car owner's manual will usually produce a nice little chart to show this... because the oil you use in Boston is not the preferred oil for Death Valley.
I agree, for a walk-behind that's more than enough. You're probably not getting more than 40 hours during a mowing season. Just be sure to change it right after you mow the lawn, while the oil is HOT ! The difference in the newer oils (versus those you may have grown up using) is that all the trash is suspended in the oil due the detergents. To get that trash out of the engine you'll want to empty the oil while all the bad stuff is in suspension.
I personally find tipping the whole mower to be much more efficient way than using the old drain plug.
Hope this helps.