YOU guys up north really do need to worry about oil weights to keep you cars running. Down here in the south, you probably are correct that it is more about "babying" our cars.

It rarely get below 20F here (-6.7 C).

Later.

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1993 W124 300D 2.5L Turbo, OM602.962
2000 Chevrolet Cavalier, 2.4L DOHC
2002 Ford Explorer, 4.0L SOHC
2005 Toyota Prius, 1.5L

http://www.fuelly.com/sig-us/40601.png

My vehicles run:
0W-40 synthetic in winter = below ambient 70F.
15W-50 synthetic in summer heat = ambient above 70F.

It can be difficult to start a diesel using 15W-50 or 15W-40 in Michigan at ambient temperature 0°.
Drop the ambient to -25°, and 15W-50 or 15W-40 are too thick for proper circulation/flow...
I gain 50-75 RPM cranking speed, and a quieter start/running engine by using 0W-40 in winter..

If you don't want to change the oil every season, use a diesel rated 10W-40 or 10W-50 as the MB owners manual suggests...