I have for many years used Castrol antifreeze and corrosion additive in my cars including my 3500s. I have always changed it every two years in accordance with the instructions. I recently purchased a late model Japanese car with an aluminium radiator. They tried to sell me a premixed coolant which would last five years or 200,000 km. Which is right? I have a pit in my garage which makes oil, coolant changes and general maintenance easy as I am wary of entrusting my vehicles to garages as they promise the world but do not always do things properly