Not sure about South Africa but in the US most interior and exterior house paints are water based latex paints. Oil based paints do penetrate better and are more durable but the long drying times, release of hydrocarbons into the environment, and more involved cleanup have made oil based paints less popular. You are correct that it is foolish to mix paints unless you know for a fact that they are compatible.
Regular paint? You can mix water based paint with water based paint. Paint to paint walls are usually oil based (check on the label) and you cannot mix oil with water based because it will ruin the paint.