BMW handbook recommends distilled water as impurities are removed through the distillation process. Some say de-ionised water serves the same purpose and is fine for this application. Still others will say tap water is fine and have used it for years without issue.
I say I'm not qualified to comment so I just followed BMWs recommendation and bought distilled water for my imminent coolant change. The cost difference to deionised is negligible for the peace of mind - couple of quid which is nothing in the context of nearly £100 for coolant, thermostat and belt that will also be changed.
10.5 is the capacity of the system and it's filled with a 50/50 mix, i.e. 5.25 litres of coolant and 5.25 litres of deionised/distilled/tap water
