Castrol syntec is not a true synthetic oil. It is a hydrocracked oil called a group III oil. It is better than regular dino oil but falls short of the group IV oils that are true synthetics like Amsoil and Mobil 1. Castrol syntec is overpriced for what you get.
Chevron Supreme oil is group III and costs no more than Penzoil conventional oil.
Group IV oils can take higher temps than group III and lower oils. This is why so many turbo owners use it. Turbo bearings can cook the oil evaporating it leaving solids behind on the bearing. This is called coking and shortens the turbo's life. This is the main reason I picked Mobil 1.
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