If you were using 10W-40 and that ran fine and now are using 10W-40, then the different oil is not causing your problem. When the weather is cold, the lower viscosity number is what matters; it's the number that tells how thin the oil will stay when cold. The higher number tells how thick the oil will stay when hot. So your lower number - 10 - has not changed and thus your cold weather performance should not be affected either.
Perhaps there is some other temp-related sensor on your car that is showing its weakness now that you are in colder weather. I have a Ford motor in my brick, so I'm afraid I'm not very knowledgable about Volvo motors.
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'84 '285' rice-eater with lotsa hp & performance goodies
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