0W, 5W, 10W, 15W have defined viscosities at defined temps-so 15 W will not match these. The viscosity 0f a 0W oil at -30 C (3250 cP) is about the same as 15W oil at -15 C (3500cP). Now if you are talking a 10W-30 oil (conventional) then there isn't a lot of difference and probably at about -30C (-22F) there won't be a lot of difference between the 10W-30 Conventional and 15W synthetic. But they are both pretty much useless there-or close to it.
If you are talking a straight 30 wt. then the 15W-50 will be waaaaaaaaaay thinner at lower temps.
[ June 17, 2003, 08:26 AM: Message edited by: Al ]