Rearend oil weights and MPG

I put synthetic gear oil in the front and rear diffs on my Jeep, I got mayybe 0.5-1 MPG improvement. I didn't do it for fuel economy as much as helping things to last longer and stand up to abuse from occasional off-roading.

I have a 1.5 quart bottle left of the synthetic stuff and 2-3 quarts of conventional, I'm not gonna bother when I swap the 3rd member in my Duster just use the rest of the synth, add friction modifier and top it off with conventional. It doesn't get driven enough for it to matter plus the rotating mass of the 8 3/4" is already huge for a car that size, better gear lube won't do much.