you should definitely see improvements in both economy and power. 25mpg might be a little optimistic though (unless you *drop* the suspension and put some skinny, high-pressure tires on).

nearly two years ago i put 75w-90 amsoil in both diffs and the transfer case, 5w-30 amsoil in the motor, and amsoil ATF in the tranny. mileage improved slightly...from 16-18mpg to 18-20, about 4-5%. myself, i wouldn't mess with fuel additives.

i don't have personal data on power improvements, and i know there is some skepticism that oil can add power, but i did see an article in 'Roundel' (the BMWCCA magazine) a while back that documented this, at least on a little 4 cylinder.

they dynoed a 318ti with petroleum lubricants, then drained it and refilled with equivalent weight mobil 1 synthetics, and dynoed it again. this is a ~140hp 1.8l motor, with a manual tranny and LSD. my recollection is that they made an extra ~7hp.

in the monty, it would seem to me that with bigger parts and more gearboxes, the frictional losses would be higher. so hopefully the gains from synthetics would be higher as well.

finally, stay away from the K&N. it flows better, and will improve milage slightly, but it doesn't filter worth a damn.

fwiw,