Places like chevy dealerships make it worth it to go every 3,000 by offering oil change, fluid checks, and full inspections and tire rotation for $59.00.
I commute over 1000 miles a week. At that rate, actually about 70,000 miles a year, 70,000/3000=23. 23 X 59 = $1357.
I presently spend about $140 with 2 Amsoil changes in that period and get about 2 MPG better with my Focus than it "should" get. Plus all the time saved.