Like most here, I stick to a 4-5k interval. If I am heading out for a road trip, I will get it changed early before hand, if it would be later than 5k when I return. And when I used to autocross, I'd get it done at 3k, no exceptions.
I'm also using Pennzoil Platinum, only because it's what my dealer offers. Always been a Mobil 1 guy, but that's gotten way too expensive. (Keep in mind, I am no longer doing my own oil changes due to bad back)
Had my oil changed about 4 weeks ago, just before I went to Deal's Gap. 4 weeks & 3k miles later, I am going in Friday for my next change. After running hard at the Tail of the Dragon for a week, I figure fresh oil is cheap insurance.
Previous post about deposits is one of the most obvious reasons not to increase the interval too awful much, just based on the oil you use.