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What car do you have?
I use 15w40 year round, down to about the coldest temps we get here in Tucson (which can be subfreezing, down in to the 20's, overnight) and haven't had any issues starting.
If your glow plugs are indeed pretty fresh, and working well, you might look in to the health of your battery, the condition of the battery cable (can be hard to discern since it's wrapped in insulation, but if it's of unknown age, it would be worth looking at as closely as possible), and the health of your starter. If your engine is not spinning fast enough under cranking, good glow plugs won't be enough.
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