I want to buy a split ac system but confused a bit because of its functioning, as I understand there is no ventilation of fresh air in split types, therefore there might be a lack of oxygen when running for a longer period, while in window ac because it sucks air from outside there will be more fresh air.
Is it right,please help me out.

sorry,,,whoever gave you info doesn`t understand the system either,also a window ac doesn`t pull its air in from the outside
you need to call a HVAC contractor to come to your home and while he is there working on you a price ask him to explain how everything works,,then you can decide for yourself,,,,,,,,,

with your window ac running take a tissue or light weight paper towel out and hold it against the vents,does it suck in or blow out? Window ac`s suck in thru the filter at bottom part and blow the same air back out at the top