Yep G-Man II speaks the truth.
In fact, I would suspect this has a lot to do with it's popularity among other things.
It probably stays "thicker" longer than some 40 weights.
Castrol Syntec 10W-30, which I like, runs almost in the upper 20 wt. range. I'm running M1 10-30 now also. Rest assured that 10-30 are not higher in viscosity at operating temp. than GC.
Don't let the 0W-30 designation fool you.