It has been cold highs in the 5F to 8F colds in the 5-F to -16F.

I have a heavy foot year around... Summer with 95% 2 wheel drive I get about 15-14 mpg Not all the bad. Winter with say about 75% 4 wheel I get about 13-12 mpg and This last tank I did 4 wheel lock all the time and I get like 10-9 mpg. I know I let it idle a bit in the cold weather. But lets say like 1hr total on a tank of fuel. SO I don't think that's why its so low.... Might the gear oil in the diffs be affecting my MPG?? It has been cold highs in the 5F to 8F colds in the 5-F to -16F. I put a have a 195F thermostat, and running 5w-30 oil with Lucas stabilizer. I just saw today they say to run the full synthetic stabilizer at temps below 5F. So I will change the oil and run that in there. Might I get some better mpg with diff diff oil?? I was thinking of running the full synthetic Lucas stabilizer in my diff. The other deal is I am thinking should I get a Wide Band and see if I am getting a good A/F ratio. This is a dual use.. I want to put a MS setup on this and possibly get a Turbo. I know Taking it ez would help.. when the wife drives it she gets about 1.5 mpg better but that only puts it at like 12-11mpg now. I am not sure how well the stock tune is for my motor. I have Fairly extensive head porting and chamber work, manifolds all port matched as well.

I think the Montero get's great milage for a 4x4 that size though.