Well one of the differences I think is worth noting is there is keeping an engine clean and ability to clean. I think a robust synthetic from any brand will contain detergents to clean up some % sludge. However, to clean up carbon deposits takes a good amount of solvency to break down and that is where things get interesting. It appears the oils containing some form of group V base oils handle this better. But even within the V range, some are known to clean better than others.
My comment was in reference to the 14%, which if it's the same test I'm thinking of (D6593), is not that that great. Amsoil claims 90% better protection against sludge.
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