Considering there is no good synthetic 5w30 oil in USA (exept for Amsoil) 15w40 ...
Real synthetic oils available in USA include (but not limited to) Castrol Syntec 5w40 (formulated for Volkswagen turbo engines), Mobile1 0w40 (European formula), Amsoil (all of the grades), Dello400 synthetic 5w40 (Diesel) and Shell Rotella Synthetic 5w40 (again Diesel).
Every other "synthetic" oil on the market IS NOT a true synthetic (Mobil1 being Class4, comming realy close to a true synthetic, but still not as good as Class5 oils listed above).
Rubo, you did not provide a definition of a "real" synthetic oil. Usually, it means that the oil base is pure syntehtic, plus some additives. It's a myth that all good Europian oils are "pure synthetic". They just have more additives than an average USA oil. I had a book by a Russian oil specialist with detailed study of the modern oil technologies. You will be surprised but an average good Europian oil has a rather mediocre oil base. They just add good extensive additive packages.
More narrow definition of a "good synthetic oil" is "oil mostly of 'ester' base". I am almost 100% sure that no oil listed by you has mostly ester base. There some Europian brands with ester bases but they are really expesive.
And you know, there is an ester-based oil in the USA. I mean the "Red Line". I used it, it was a great oil, but I just did not feel the difference. Our motors are too simple for those sofisticated oils.
As for the 15W-40, take into account, that if you have more additives, then you have less base (which is the "lubricating" part of oil). So, this is a double-ended thing.
My engine works fine with "simple" Mobil1 5W-30.
