My dealer said that it only needs an oil change every 13,000 miles. Is that really true?

No. They require 7500 intervals but most choose with synthetic to do 5000

If you use Amsoil its ok.Mobil one even uses regular oil as a carrier.

Recent Mercedes have an on board gadget that monitors oil quality. You will get a service reminder on your dashboard information center when you need a change. Intervals can be as long as 13K miles. When you have your oil changed, you MUST use synthetic, and your service indicator needs to be reset.

this depends on many factors, such as how hard you drive, conditions of engine, and blah blah... for the most part it is really up to you. i would recommend to change your synethic oil about 5000-7000 miles.

Technically YES. I own a four cylinder turbo charged sports car I use Mobil one synethic.. best thing you can do for a car!! I still change my oil every 3000 miles in my turbo. and In my Toyota truck I use the same oil but change it every 5000k some might say it's a waste of money, but your car will last forever! Hint.. always use a Mobil One oil filter it has a built in gate that keeps the oil from going back down into the oil pan when not running. that way there's always oil in the top end when you start up.. It's starting an engine that kills it over time