Its never to late to switch to Synthetic. The benefits in a turbo car will far oughtweigh any downside.

The reason why people say not to switch from regular to synthetic is because synthetic oil does not cause o-rings to sweel as much. This means that in an older car, you may cause leaks to develop. But in reality newer synthetic (incl. Mobil 1) have additives that pretty much make that problem a non-issue.

Mobil 1 10w-30 is the way to go.