2006 - 190k here. I check oil every 500mile; roughly add a quart every 1000 miles. I drive 2k a month so maybe 5-6 quarts in my case. 15 quarts in 3 months seems excessive. I had a dealer not attach the upper intake manifold to throttle body properly burned a ton of oil before I found it.
My coil started to fail very similar to the symptoms you describe @120k, then one morning it was misfiring like crazy. I'd check coil for nice blue sparks.
