It was just the two changes so far - ~6500 mi (4.5 months) on factory fill, ~8100 (4.5 months) with the Castrol syntec. Book is 3750mi for severe, and 7500 for normal. Mostly longer trips, but not on interstates (30+ mi at a time) - with a few short hops thrown in. To give you some idea, my avg...