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I would not mix a synthitec oil with a non synthitec oil Isn't all Royal Purple oils synthitec? I like non synthitec Valvoline and if it is a flat tappet motor a bottle of either cam break in additive or Zinc additive



Brad Penn is semi-synthetic......not sure how that plays mixing with a synthetic(or non-synthetic for that matter) Either way I agree with the why even change it crowd.