Hey, I haven't really kept an eye on my exact fuel economy, but I also have a 97 with 32's on and use mobil 1 10W30 in the engine as well. I have factory Mitsu fluids in all the other systems from my 100k service. I would have to say that mobil 1 in the engine is a wise choice from 200k miles of experience on our old '91 Montero, but I don't know about any benefits from putting synthetics in other systems of the vehicle. If I determine my gas mileage, I will let you know, but I thought it was better than 18.5 on the highway. Good Luck.

Scottie

97 Montero SR w/Mitsu Adj Shocks & Rear Diff Lock ARB Front Diff Lock 4.9 Ring/Pinion 3.15 transfer box 43:1 crawl ratio 2" suspen lift 2" body lift 2" Gas Tank Lift 35x12.5 MTRs 15x10 Wheels Rock Sliderz ARB front bumper w/12k winch Custom rear bumper