Molecular weight is just an older term for molar mass. They are the same.

which is the average molecular weight 603 Mobiltherm?

i got 23765.4

The average molecular weight of dry air is 28.96 g/gmol.

Simply looking at the expressions that derive weight average and number average one can see that the weight average is a second order expression where number average is a first order. To put it more simply: weight averaged molecular weight emphasizes heavy molecules more than light ones; number averaged molecular weight says a molecule is a molecule and counts them both equally.