Ohhhhh! This should spark quite the debate!
I, personally, also use Mobil1 on my car, and I change it at the 3000mile marker simply because it's a turbo car, I boost fairly frequently, and I drive a lot on the highway at sustained higher RPM than most people.
Now, this is the part where it's going to get interesteing...
Synthetic oil by nature, is designed to break down slower than conventional oil. It's also designed to and does stand up to heat way better than conventional oil, as well as having certain characteristics that allow it to lubricate more efficiently. Now, because it doesnt breakdown as quickly as conventional oil, you can typically get away with a longer interval between changes. Having worked in the automotive repair field for over 10 years, I've seen multiple vehicles who use synthetic oils and easily roll over the 7k mark before coming in for an oil change, with no ill effects. BUT, these are STANDARD, normal-person driven vehicles, NOT performance vehicles that are known to produce excessive heat/etc.
MY basic rule of thumb goes like this:
Conventional oil change (regardless of driving style, except when "beat on" constantly): 3k miles/3 months
semi-synthetic: do not recommend
full-synthetic(non-turbo/supercharged/forced induction/performance vehicles): 6-7k miles based on driving habits/etc. Normal city driving, NO towing, limited/lower amount of highway use/sustained higher overall RPM
full-synthetic (turbo, HI-PO, forced induction/etc): 3k miles when driving styles/conditions permit. I.E. sustained higher RPM usage, "spirited" driving/etc.
Now, this is simply put MY OWN habits as far as oil changes/etc go. Im sure someone else will come in and give you a completely different opinion, as this seems to be one of those topics that there is no definitive answer to.
In a nutshell, just keep the level correct, and the oil/filter itself clean. If when you check your oil, it smells like there is fuel in it, it's abnormally dark/black/thin, it's in need of a change(except where diesels are concerned, which is a whole 'nother ballgame).
As far as taking your vehicle to get the oil change done, thats all really a matter of preference. Allthough, I will definitely tell you supplying your own oil and filter and paying for the labor alone is by far the cheapest way to go.
I hope that helps!
-Jake