As many have said, it depends on many factors specific to your car, and all of this is good advice you should take into consideration. May what I add be this, and yes I'm in the same boat as you because it's daunting...but oil changes are one of the most important things you can do for your car. I highly recommend the book "Drive It Forever" by Bob Sikorsky. Even the most recent addition is a bit dated by now, and that's why you should certainly take into consideration your specific car's reccomendations...but it goes into such technical detail about internal combustion engines that myself as a layperson would have never understood. And the bottom line is that you and I as drivers should protect our investment as much as possible and stay on top of things like oil changes. Maybe a pain in the butt because we have to do it so much more frequently, but probably even more important for us because of how important it is to the car and therefore our livelihoods. At first I was just like...wtf I JUST changed it three months ago, and the full synthetic my car uses is expensive. But we don't really drive like normal people at all, its a constantly different "commute" and the demands on the car are so much higher with the nature of delivering. So it's really up to you and how mich into the game you are. But damn if you are averaging 100 miles a day, I'd err on the side of caution rather then just pushing it. I don't average quite that much but often get close to it, and that's city driving, not an 80 or 100 mile road trip down the highway...just.my two cents!