Originally Posted by SouthArk370Z

The acid doesn't collect if it just sits there without the engine running but it will over time. Every time you run the engine, you are adding a little acid. Don't go two (or more) years before changing the oil.

Water is a product of combustion of hydrocarbons (hydro = Hydrogen; Hydrogen + Oxygen = water). Piston rings aren't perfect, so some of that water slips into the crankcase. PCV certainly helps but does not totally eliminate the problem, especially if you make short runs where the crankcase doesn't get hot.
If your car is not stored at a constant temperature, the crankcase will breathe as the temp changes. This will allow some condensation to form.

Neither is as big of a problem as it used to be (buffering agents in the oil plus PVC) but they have not been eliminated.