Depends on how cold it gets where you are. If you must make subzero starts (minnesota, for instance) the 5W40 will help considerably. If you only get moderate winter temps (teens, for instance), the 15W 50 will be fine.

Synthetic oils have different viscosity characteristics than dino oils, and the viscosity ratings don't really reflect the flowability at low temps -- there is much more difference between a dino oil at 5W and 15W that synthetics at 5W and 15W, and the 5W will be MUCH thinner -- 5W30 Mobil 1 pours almost like water!

Peter

__________________
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!