whats the best cooland i should buy? im flushing my system as i changed the thermostat a while back and never got round to it and now my coolant is old so time for a change. Ive been looking at bmws own coolant £10 for 1.5 litre. How much do i need? i believe its a 4 litre system, and sounds a bit daft but i have to water down the coolant right? lol sorry never done coolant before. cheers