Here I go, walking upstream again...
I will use 10w/30 while my vehicle is under warranty and after about 50k miles I start using thicker oil. Since I already have 10w/30 and 20w/50 both on hand, I mix them about half/half.
My 99 S10 ZR2 truck is getting better mileage on the half 10w/30, half 20w/50 mix oil...not sure how to explain that though.
I have asked oil mfgrs about mixing oil wts and they all said the same thing, they make most any wt I could want, but mixing won't hurt. As far as the resulting viscosity value, it may not be an exact mathematical average but is close enough.
You could also say that you get the best characteristics of both oil wts...seems to be working for me, has for years.
Far as I'm concerned the xw/30 oils have been a CAFE requirement, and is a marginal wt oil. I am more concerned with engine longevity ..but to each his own
I hope nobody on this board is in New Orleans, good luck to them if they are..