Electrolysis and the Fluctuating Ammeter

Old coolant will show more voltage than fresh coolant even in new cars that have a much different system ( more grounds routed back to the battery via a wire ).
They say one can test coolant with a volts meter. More than 4 tenths of a volt at 2000 rpm means replace the coolant. I don't know if one could get a reading that low in a classic car.