15w50 is better for cars like Scott's, where 70% of the miles on it are full throttle on racetracks and autocross courses. 10w30 is more suitable for daily drivers in the summer, 5w30 in the winter, unless subaru has added 5w20 to owner's manual. Then I would run that.
Dealers won't care what brand of oil you bring them, as long as it's the right grade and has the proper SAE requirements met. They won't flinch at the Mobil 1, especially if you ask for an OE filter, which I personally would recommend even if you change them yourself.
I used Mobil 1 exclusively in my RS, except for maybe 3 or 4 oil changes where I used Redline. Sometimes 5w30, sometimes 10w30. When it was totalled I had 106k on the odometer, and it was using less than half a quart between my 6000 mile oil changes. Most of that was probably blowby from the PCV system. And I certainly didn't baby the engine in that car from behind the wheel.
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FWD is the new AWD
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