I'm going to assume your thread title had nothing to do with a fuel question, because I didn't notice you asking what kind of gas to put in the car. All I saw was info about a car you're intersted in, so I have no idea where this stuff's coming from.
Heed the advice you've been given here about buying a Boxster...there's been plenty of it. For what the dealer's asking you could find yourself a younger car with fewer miles.
Do not purchase any Boxster without getting a PPI done
yourself, meaning you getting a mechanic who knows these cars to come and look at the car inside and out. Yes, you'll be paying for it, but it's a fraction of the total cost of the car you're looking at and you'll be glad you did it rather than buy a potential bucket full of expensive problems. Those who do PPIs know what to look for and
will find the issues if they are there.
These cars can be reliable and fun if well-maintained, but as it can be with any car, some are not kept well and can simply look pretty while being held together with duct tape and zip ties. Find a car that has maintenance records and a clean title. I know this car caught your eye and you want it to be the right car the first time around, but you won't know that for sure until you ask the right questions. The dealer just wants to sell the car and they will take advantage of your lack of car-buying knowledge, telling you anything you want to hear until the car's in your hands and out of theirs. If it's not a Porsche dealership they're even less likely to honor items under warranty, and given the age of the car you're not going to get one for more than a couple of months, if that, depending on the laws in your state. Know what you'd be getting into ahead of time rather than later.
I don't have it handy, but it would be great if someone could post a link to Mike Focke's site for you to refer to. If someone doesn't end up doing it I'll come on here later and post it. Read his site, he has a ton of useful information that you should read before buying a Boxster that I wish I'd found years ago.
Don't get hooked on the car, because you haven't bought it yet. If it turns out to be a problematic car with a lot of issues that neither you nor the dealer will be willing to invest the time and money in fixing, be prepared to walk away. Keep your emotions in check so you can evaluate things clearly. There are several hundred thousand Boxsters out there that are waiting for you to spend your money more wisely on them.
For that matter, you're young and have plenty of time to find the right one. Trust me, they're not going anywhere, and if you have to take a plane to get one somewhere else in the country and drive it back home, it will be worth it if it's the right car for the money. There's even a For Sale section of this forum. Maybe you should take a look around.
EDIT: A perfect example of what you can get for $11k (it's been sold, but note the info about the car and what it came with):
http://986forum.com/forums/boxsters-sale-wanted/34621-2000-boxster-w-61k-miles.html
Yet another, older with lower miles, older gentleman's weekend car:
http://986forum.com/forums/boxsters-sale-wanted/34158-1997-porsche-boxster-88k-miles-%249800.html
Good luck, and don't hesitate to ask us more questions, but do try to take our advice with more weight than what the dealer tries to tell you. They're in the business of making money. We're in the business of making sure potential new Porsche owners don't get ripped off or stranded on the side of the highway.