No engine manufacturer (including Mercedes) recommend/allow extended oil change interval while using Synthetic oils instead of conventional oils..
Your car has been performing well for several years with dino oil, don't expect it to perform better (or worst) with synthetic.
There has been some very well designed and very successfull marketing campain trying to push synthetic oils in the market.
Synthetic oils are good and perform better in certain extreme conditions, (very rarely encoutered in normal driving) but I rather have my engine bath in frech mineral oil than in 10,000mi old synthetic oils. If you don,t mind the cost, spend the same money in more frequent oil change. Your engine will thank you over the long run
Just my informed opinion.