As others have mentioned mix away.
I usually mix to use up what I already have.
But blindly thickening up an oil to gain "additional protection" makes no sense to me.
And as far as following the guy who "knows his stuff"; does he know more than the engineers who spec'd the oil for your ride? I don't think so. He may have a very valid reason (although I doubt it) for doing what he does, but you haven't mentioned it.
Personally I can't think of a worse reason to deviate from a manufacturer's recommendation.
I usually mix to use up what I already have.
But blindly thickening up an oil to gain "additional protection" makes no sense to me.
And as far as following the guy who "knows his stuff"; does he know more than the engineers who spec'd the oil for your ride? I don't think so. He may have a very valid reason (although I doubt it) for doing what he does, but you haven't mentioned it.
Personally I can't think of a worse reason to deviate from a manufacturer's recommendation.