Will 75-140 gear oil reduce gas mileage 2mpg?

[Jakede]

Hi all, I recently did a fluid change on my 05 silverado 2500hd. (4.10 rear diff, auto 4x4) I went to the local dealer who persauded me to buy there "4x4 service kit". This includes BG synthetic 75-140 for the front and rear diffs, and 75-90? for the transfer case. (the transfer case weight escapes me at the moment...) I also changed the tranny fluid/filter. My mileage went from 13.2mpg average to 11.5mpg after doing this service. I have checked it over 2 tanks worth of gas now. Has anyone experienced this? Thanks for input! Jake

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[acnas]

75-90 synthetic front and rear diff. Auto trac in the transfer case. I wouldn't drive it with what you have in it now.