There is some disagreement about calling hydrocracked crude oils " Full Synthetics" . I understand, as a result, Castrol Syntec synthetic is not allowed to be sold in Europe as a full synthetic. For a time, Mobil 1 was reportedly the only major U.S. brand that was a true full synthetic. Lately, even Mobil seems to be a bit vague as to the composition and origin of their synthetics.
Overall, however, the source may not matter, since all of the synthetic types seem to perform well in actual engine tests, except for one or two private brands who choose not to submit their oils for standardized testing.
When I was in the fuels and lubricants business, it was fairly well accepted that all of the major brands of conventional mineral oils performed about equally well. The exceptions were batches of oil where something had gone wrong , but that information was usually well guarded. Consumer Reports ran some oil tests a few years ago, and as I recall they found some national brands " sheared down" to lower viscosities sooner than others. Shearing down is considered bad. Castrol conventional oil came out looking good in the Consumer Reports tests. Of course, test results can change rapidly, as new test standards are introduced.
