I currently use 15W-50 Mobil 1 but back when Mobil 1 10W-30 was a true synthetic I did a little comparison test. I had two Mercedes (one diesel and one gas). I put 10W-30 in one and 15W-50 in the other and then paid attention to the maximum coolant temps and oil pressure at idle. In every case the heavier oil produced lower coolant temps and higher oil pressure at idle.

I then reversed the procedure at the next oil change and the heavier oil still produced better results.

I lived in Minnesota (USA) at the time and found that even in sub-zero temps the 15W-50 would pour more easily than 10W-30 petroleum based oils.

Now that I live in Arizona the choice is a no-brainer.

Both Mercedes and Porsche recommend using 0W-40 Mobil 1 but state that you can use 15W-50 if you can't get 0W-40. In fact, when Porsche first started installing 0W-40 Mobil 1 at the factory, I asked a U.S. owner what he was told to use since 0W-40 wasn't available at the time in the U.S. He told me that he was told to use 15W-50.