Castrol Syntec 0W-30 is very easy to find in USA. They sell it at my local NAPA store here in Chicago. I also see it all the time in Autozone. You can even buy it online on Autozone!
As far as Mobil1 goes, I have some real world experience with it and I can tell you right now that this is one of THE WORST synthetic oils that I've ever seen. Its not even Grade 4 synthetic oil.
Here is the summary of the oil analysis that I did a while back for my VW. The analysis was performed by BlackStoneLabs and if interested, I can provide you with all the original reports from the laboratory.