OK. I'm raising the BS flag. 15W-40 has been around for ages and has successfully been used in many engines with out any issues. I agree that 5W-40 (a synthetic) is an improvement over 15W-40 due to the fact it is a synthetic or quazi-synthetic and it also has a lower pouring point (5W vs 15W). The fact remains if it gets that cold you must use a block heater that will keep the block and hence oil warm enough that it won't be an issue. If it's that cold glow plugs alone won't work. You guys split hairs over which brand to use. Then criticize them if they don't use your brand and these oil wars start use this, no use this... USE A PROPERLY RATED OIL FOR YOUR ENGINE. DIESEL ENGINES REQUIRE "C_" Rated OIL NOT "S_" Rated.
Thank you for allowing me to RANT.
15W has a pour point of Appox -30 deg C
5W has a pour point of Approx -45 deg C
Pour point: The pour point is the lowest temperature at which an oil will flow. This property is crucial for oils that must flow at low temperatures. A commonly used rule of thumb when selecting oils is to ensure that the pour point is at least 10°C (20°F) lower than the lowest anticipated ambient temperature.
http://www.engineersedge.com/lubrication/pour_point.htm
In short... 15W is good to about -20 Deg C or -4 deg F