The hypermile folks swear by Mobil1 for some reason, and even they say it only makes a small difference, in the 10ths of a mile per gallon. I used Mobil1 in my STi once, never again. It went through 1.5 quarts between oil changes! It usually uses about 1/2 quart.

I don't see anything wrong with using 0W-whatever so long as it is quality stuff. Sounds like an especially good idea in frigid weather.

I personally run Pennzoil Platinum in the Subie (5W-40) and have been running the same stuff in 5W-20 in the Fit. So far so good I guess, no ill effects certainly, but no huge leaps in MPG. Not that I'd expect either.

 

Castrol is the best synthetic by far honestly. I run non synthetic 5w-30 cheap stuff and get 32-36mpg in an Auto Sport. I run Marvel Mystery Oil and Lucas Oil Treatment with 5-30 regular oil. Runs super quiet. Last tank was 36.6mpg mixed traffic.

 

dang..not bad!