Well, -44F is about as low as you can get in the typical oil testing lab (diesel fuel usually doesn't have a pour point less than about -30), and "pour point" is the temp at which it will visibly move in the test aparatus in either 5 or 10 sec if I remember correctly. Long time since I did any of that stuff. Usually tested by chilling the oil or fuel in a brass tube submerged in an acetone/dry ice mix.

The lower number is the cold viscocity, the higher number is the hot viscocity rating. May and may not be all that different for a 15W and 0W synthetic, since they don't change viscosity all that much anyway.

The visible difference I've seen is that the 0W oils pour almost like water at room temp, while the 15W-50 pours more like oil. I'd assume they have similar characteristics at very low temps as well, with the 0W-40 being "thinner".

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!