Anybody using synthetic oil? Help with MPG? etc.

5W-30 to 5W-50 all have the SAME viscosity at room temp...5W

It is how the viscosity changes as the oil temp increases...they all
thin out (lowers viscosity) as their temp raises

The difference is how that curve looks over its temp range

At a top temp. A 5W-30 will have the viscosity of a single
30W oil. At that same temp, the 5W-50 will have the viscosity of a 50W oil

Again, both examples at room temp will have a viscosity of 5W.