1. Air intake has engine breather tube. 2. Gases from engine contain vapours from combustion. 3. This condenses inside the inlet manifold creating an oily liquid, over time it accumulates enough to pass the gasket. Place your hand underneath the back of the inlet manifold, if it's wet with 'oil' there's the problem. Ideally set up a catch tank to contain this before it enters the inlet manifold, they are cheap enough. This is mine on the 2.5v6 (which is easier to do).