It depends on the duty cycle. My old commute was on the poor end as it wasn't long enough for thorough warm-up so the naughty stuff didn't get evaporated off. My Mom's commute was absolutely perfect though- about 15 miles of lonely straight level 2 lane one way with only a couple stops the entire trip. The cylinders in her car at 150,000 looked as new- no ridge at all and crosshatching looking fresh. Her car didn't get the "severe service" oil change regimen either. If I had that sort of duty cycle I would change the oil even less often than now. I've only recently put synthetic in one of my vehicles not for any oil longevity reasons but mainly for flow at -40F cold starts. I believe I hurt my '94 way back when it was nearly new with one of those cold starts- ever since there's been a slight lifter tap on cold start.
