Castrol synthetic or Valvoline?

FWIW, about the only people who think that the Castrol synthetics are actually synthetic is Castrol, and the Supreme Court of the USA. Other than that, it is a mineral oil. Castrol paid a butt load of money to get the result it wanted.

That's all I wanted to point out.

For the nattering naybobs and flat earthers, I can send you the link where it is discussed by the single smartest tribologist I know.