When I change (or have changed) the green coolant out in my 300SEL, should I just simply drain out the green until it stops, then refill with something like Zerex G-05, or is a flush necessary? I have heard from some that flushing on older cars could be a mistake because if you have a weak headgasket or heater core, it could cause problems, but I have no idea. My car does not have the original radiator if that makes a difference.

Thanks.