Freeway miles are easy on an engine and oil. It's the frequent start stop city stuff where the engine never gets up to temp where you have to worry about the oil sludging up.
I wouldn't sweat going 15k with mobil 1. We got ~230k on the wagon before the boy decided to play rally driver on an icy road last winter. Regular intervals were 10k, a few times stretched to 12+k and while the engine burned some oil and seemed to be a little down on power from age, never an oil failure.
