You are confusing hydro oil with engine oil.
Engine oil gets contaminated with carbon from the combustion process which is why it turns black
On top of that there are acid byproducts of the combustion process and of course the tiny amount of oil that is on the cylinder wall gets the whole whack from combustion, before you start on consumption of additives & physical deteriortation of the oil itself
Thus it has a fairly short working life
In a hydro you get contamination purely from wear + brake debris which is a tiny amount and mostly filtered out on better brands
So the determining factor is the mechanical breakdown of the oil itself.