Mobil 1 = bad gas milage?

[limeylee]

Just changed the oil for the first time in my 03 Denali.
Put mobil 1 Synthetic in it and have noticed about a one mile per gallon drop in average milage.
Should this happen? don't know which oil was in it before?

 

[boostaholic]

Oil shouldn't change it that much, I actually gained some power and smoother idle with my royal purple but I've never lost mileage from running oil. Seems like mobil 1 has been slacking lately and they aren't quite up to par.

 

[YAYSTA]

thats strange, but like he said m1 has been losing quality latly. did you do anything else to the truck? or just oil change

 

[oomindwarpoo]

Maybe you put the wrong viscosity..

 

[Darkitec]

I lived in Arizona when I got mine. Gas changes in Oct thru March to horrible oxygenated blend. Had an abrupt drop in Mileage at that time 3 to 5 mpg. Did Cal change their gas recently?