15w is too thick for the cold. Don't you watch the commercials about how most wear comes at start up? Do you really want your motor to lack lubrication when it is cold?
Here is a simple test, buy a bottle of 15W40 and 5W30 and 0W20 of the same brand, then pour it and see how much different they flow and you will see the difference.
I hit 240F in traffic before the oil cooler
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