radiator warer refilling questions

[tablahan]

My friend noticed that my ML320 needs a little topping off on the radiator. Obviously he opened the radiator cap to see this. Now i used to own BMW and the BMW dealer said that i should not open the radiator because air bubbles come in and the water should be enough til the next service (a few years later)....

Question 1. Does this apply for the ML as well?
Question 2. If it doesnt apply and i can open the cap myself, can i top off my radiator with water or must i use antifreeze? What kind of anti freeze (grade brand, etc)
Question 3. If i cant open my rad cap because air bubbles will form how do i remedy this do i have to take the car in for service?

Thanks in advance!

[tabo]

I believe it is not necessary to check your coolant from the radiator cap, but by looking at the coolant reservoir. if its below the "LOW" line than you should add coolant mixed with water(50% coolant, 50% water) for the ML.. I maybe wrong thou...