Spray that nut with penetrating fluid e.g., wd40 good and soak it completely. Then leave it overnight and the next day, welcome it with an impact gun. The little bugger should come right off. Regarding your coolant question, Zic pre-mixed one comes pre-mixed (hence the name) so you don't have to add water to it. Just put it directly. My car took around 6 litres (more or less). So buy yourself 2 bottles of 4 litre and you should be good. Initially it will take only around 3-4 and the radiator will seem full but once you bleed the system completely, the level WILL go down so you will have to top it up. Additionally, it will be pretty normal if the car seems to be "reducing" the coolant in the radiator. It will not actually be reduced rather the air bubbles hiding deep inside the cooling system will gradually come on top as you drive the car around. Mine kept on reducing the coolant for 3-4 days after first fill and I kept topping it up. After that, it's totally stable. Not a drop is reduced in the last 3-4 months. However, the pre-mixed green stuff needs frequent replacement i.e., every 8 to 10K miles. I am ready to do it because I don't trust the so-called long life ones available in Pakistan anyway. As per me, it's a good idea to flush your cooling system frequently and adding in fresh coolant every 8-10 k miles.
