Hi Syl... I defo think most of that is water vapor in your exhaust gases and you most certainly are using it some where if you have to top up 1.5 lt ....is there any white/grey gunge (emulsified oil) in either your coolant or rocker cover if there is, that would defo point to hg issues .....smell your header tank for petrol/exhaust fumes or if it is over-pressurizing , check head bolts
certainly sounds like hg failure.....needless to say it can only get worse if it is
