Do you use the (16lb) radiator cap that comes with the Fluidyne or the stock Mazda cap?

 

I really dont think it matters. If I were you go to discount, and get the High quality "Stant Radiator Cap" with the Red lever to release the pressure, these are the best caps, I personally have two of these on my Turbo II.
But a friend of mine had two also on his Turbo II, and he told me his car was running hot, because he had the two caps with the red handle on at the same time. So just use one on the top of the Radiator Neck, and a normal cap one on the bottom of the radiator. Also dont you dare add any garden hose water, or tap water for that matter. As tap water has alot of calcium in it, at which as the time passes by it will start to clog your radiator, and damage your water pump after awhile. So just add Distiled Water, and Coolant, and you will be fine.

 

Depends on where you live, in NYC you can get away with tap water, ours is the best in the country