You may find the technical answer on a search on this site, buy I did read a while ago that Castrol was sued by Mobil and others because Castrol was not a true 'synthetic'. Apparently, Castrol is not using the same base as other makers of synthetic oil, but is using dino oil and doing different processing to it.
What I also remember is that Castrol won the lawsuit, which is real disappointing.
I don't know on what technicality they won, but it is very clear that their syntec is not 'synthetic' in the way that it was meant to be.
I've heard so much good mentioned about Amsoil and Mobil that I would use one of these. Amsoil is expensive and harder to buy, but I have technical documents they provided to me that shows their synthetic compared to all the others under all sorts of elaborate laboratory testing, and Amsoil outperformed the others. I also believe reading they invented synthetic oil.
I hope someone with more of a technical background can comment further on this...

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".