Eric:

I'm located in southern Ontario - and also had a block heater installed on my engine - S70 non-turbo - I run 5w30 Mobil in all weather conditions (summer + winter) since the specifications of this synthetic oil exceed both film strength and pour point of conventional oils in both 5w30 and 10w30 at high temperatures - This is also true of the newer 0w30 Mobil 1 - though I'm not willing to tempt a warranty issue by going that far away from the "officially recommended" oil viscosity chart in the owners manual.

My dealership only stocks 5w30 (Quaker State) synthetic for owners requesting a synthetic oil change.

BTW - I'm a nut for block heaters - I've found that any cold starts under +5C without a block heater cost more in fuel than the equivalent 4 hours of electricity and the not-so-cold start - and that was at fuel under $0.60/L. The instant heat is a nice bonus(wife)- as well as the knowledge that the warm-up fuel-rich condition isn't washing the oil film from the cylinder walls - or contributing to fuel dilution of the engine oil (worst on carb'd applications :) ) and condensation build-up from short run cycles in cold weather.

I was born and raised in Northern Ontario - so old (good) habits die hard.

MAT