Most quality full synthetics of 5w-30 and 10w-30 have nearly identical viscosity at operating temperatures. So 5w-30 is only important for the first couple minutes after starting up.

I myself run 5w-30 all year round. But the most important part is I run AMSOIL 5w-30, which is VERY VERY different from Castrol, Quaker State and to a lesser extent, even Mobil 1 5w-30. Though the 5w-30 is often only considered a "winter" grade, AMSOIL's film strength and flash point far surpasses even Mobil 1's 10w-30.

I too start using my engine black warmer around freezing, as already mentioned, because the cost of electricity of warming the engine is significantly lower then the cost of poor mileage until the engine warms up.

BTW, just so I don't get accused of sneaking in an unwarranted push, I am an AMSOIL distributor. Although, it does say something that most of the people on the Brickboard who have done research regarding synthetic lubrication, choose AMSOIL. All you have to do is check the posts. Not everyone uses AMSOIL, but it's pretty obvious, the concencus is that it is the best (even though it may be a little more expensive).

Eric

I cannot provide you a technical answer to your question about oil viscosity. In winter, I run 5w30 synthetic in my wagon, and, as you will do, plug the block heater into a timer on the coldest nights. I've never really understood why Volvo's cold weather package (seat heaters, etc) does not include a block heater.

Neil

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