high mileage fluids

whats the deal with the oils and trannie fluids being advertised as "high mileage"? Ive always believed the theory that oil additives are useless and
theres nothing better then good old regular oil changes but the difference between your regular oil additives and these "high mileage" ones is that they are being sold by what I consider to be reputable companys(valvoline, castrol). Does anyone know what theyve added to the oil to make it high mileage? any idea what the pro's/cons of using it is?
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some other additive too but I dont remeber what it was.....
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Petebert wrote:

the oil should be a little thicker to take car of the wear in the engine.. to stay on the walls of the cylinders a little more....maybe they got something like the additives that you see advertised in TV have in them???
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That would be a change of viscosity, which is not the situation here. 5w30 in regular conventional and 5w30 in maxlife are the same viscosity.
CJ
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