My lawnmower is turning the oil into almost water, whats wrong?

I have a toro mower i bought in summer 07, when it gets hot it starts knocking, if i shut it off and try and restart it wont start. I will change the oil, by the time i am done mowing the oil is black and almost a water consistincy. I will tip it on it’s side to change the oil and it will gush out like water, whats going on??