Who makes an oil in 15W-30? I've never seen one working at Napa or Kragen. I've used 0W-30, 5W-30, and 10W-30 Mobil 1, and the only difference I've noticed was slightly better mileage while using 0W-30. But since you can't really tell how well you're motor is being protected until it's too late, I've been using 5W-30. I would guess you'll get slightly worse mileage with 15W-30, and in cold weather it might be a bit thick at first.
Khail
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