I apologise for asking this question but I have seen this happen so many times where the OEM has a generic recommendation without understanding the operation environment. So here it goes did the OEM understand the temperature operating conditions eg lowest start-up temperature, When they made their recommendation? If so 10W-40 is avaliable as is 5W-40 and 0W-40 all API rated for use in heavy duty diesel engine applications. I think the latest API designation is CI-4. If you have an older engine you won't get the benifets that the "new" formulation is designed to give you.
Vickie we have quite a few diesel engines some are operated indoors and sit idle for long periods of time in heated buildings. They don't need a multigrade engine oil because they don't have the cold temperature start-up problems. The oem's still make the generic recommendation to use a multi-grade. Some other equipment needs the 0W-XX because the -40C/F makes it almost impossible to get the engines restarted after they are shut down for a short period of time.
While I'm on the subject...block heaters (at least the ones that I am familiar with) only heat the coolant that surrounds the cylinders. This has almost no effect on the oil that is in the base pan some 10-15 inched below the actual engine block. So when the oil pump starts up the oil is the same temperature as the ambient air surrounding the base pan. There are heaters that you can get to heat up the oil in the base pan - be cautions if you go this route the maximum watt/density should be kept low to avoid burning (coking) the oil.
regards
Alan.....