I'd say that Mobil products are the only syn oils availble over the counter worth using. In the Mobil 1 line, I'd say that the 10W30, 15W50, and 5W40 are excellent, and the others are slightly less impressive.
I don't trust Castrol enough to feel entirely comfortable with their only common syn oil, German 0W30.
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Well, I have seen two engine having run on a dyno under same test conditions, one with Mobil 1 0W-40 and one with a german version of Castrol Syntec 0W-30.
Mobil 1 is good, but Castrol is (at least in this engine, within this test) clearly the winner. Better piston cleanliness, better wear performance.