I have option B installed with the dual plate, but the dealer only filled the secondary bottle to the fill line of the secondary bottle. Should the secondary resivor be filled to the top? And can i fill it with pre mixed 50/50 dexcool?

 

I would fill it up. But stay away from dexcool. Use water, it conducts heat much better. Its not cooling your motor its never going to boil. Just remember in the winter if it gets cold to add some to it do your heat exchanged doesn't tear itself apart. I am probably running 25% dexcool 75% water in my system.