From what I understand the pump blows air into the exhaust system at the cat.......just like you would blow air onto a glowing amber to get a fire started, or into a fire to raise the temp......its the same with the cat.....it works best at a higher temp, and if the outside temp is not high enough the air pump will blow air onto it to raise its temp faster so it works at 100% sooner.

As for the Mobil 1.......I think you will be just fine with the 10W30. I live in the NYC metro area, and run it all year long. If your were going to expose the car to very cold starts....like below zero....the 5W30 would be better I guess
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Steve F. in Hoboken, NJ