Universal oil viscosity?

I was in the motor oil section recently and there were two dudes there pondering all the different bottles. I wasn’t really paying attention to them but then one said something to the effect of:

“We’ll just put 10W-30 in it. That’s universal oil for everything.”

I have never in my decades of driving heard such a thing. Have you?

Back when I was a kid 10W-30 was pretty common, but the only thing I use it in now are my generator and my 30-year-old mower.

PS: I did tell the dudes that I was pretty sure 10W-30 could damage their car’s engine if it was designed to have a different viscosity.