I do it about every 3K because it is, give or take... every three months... and for $20 a pop an oil change is nothing, especially when it 'feels' better after.... and I like that. Next oil change will be synthetic, so it'll be a little more expensive, hopefully it'll give me some more distance in my gallon
6-10k miles Mobil 1. US Army motor pool changes oil based upon oil analysis. At least that is what my division did. Test reports came back fine for 10k + miles before they needed changing. And that was for SEVERE duty vehicles.
I remember reading one person who tested how long they could go - They got past 15k miles, and the wear numbers were all fine. However the original API rating of the 10w30 that he started out with had become closer to a 20w50.
And for me this is the final nail in the 3k mile "severe duty" coffin. CR did oil tests with identical NYC taxi cabs with half using drain intervals of 3k miles and the other hald using 6k drain intervals. Here is what they found after breaking down the engines:
"Even in the severe driving conditions that a New York City taxi endures, we noted no benefit from changing the oil every 3,000 miles rather than every 6,000. "
I do my Taurus, which has 149,000+ miles every 5,000 miles. It is a mail route vehicle, which mean one of the most extreme conditions you can put to a vehicle. Start and stop everyday for 700 mailboxes. I use Castrol GTX. I never understood the every 3,000 miles on regular cars, but hey...not my money. I heard from the mechanic at the Jeep dealership, that if you change the oil yourself, be sure and put in the exact oil weight as the manual (and oil cap) states or else if will void the lifetime powertrain warranty. Just what he told me, so it may or may not be true.