Hey Guys:
I believe Mobil "builds" its Mobil1 from mostly ethylene (C2H4) molecules (a gas). Syntec, although a fine product, is still a refined lubricant. Castrol finds the molecules it wants in their crude
oil and separates them out. What they are left with is what they called a "
synthetic" lubricant. Many disagree, as I understand it the courts did not.
On mixing oils the best reason not to mix brands is the possibility of the additive packages not working well together. The hydrocarbon part is close to identical. The antifoam agents, and so forth may not get along well from brand to brand.
Bob