the point of positive crankcase ventilation is to draw fumes out of the crankcase and into the intake manifold where they are burned - it helps to reduce emissions. If there is no way for the slight positive pressure to vent (as when the PCV valve is bad or the hose is disconnected) the pressure can build. the lower seals around the crankshaft are not designed to withstand any great pressure, so when the pressure builds, oil is pushed past them. |
The pressure will be less at low engine speeds and might not get high enough to push the oil out. Another symptom is that after high speed driving, the oil will continue to be pushed out even after the engine is stopped - it will go on until the pressure is relieved. but since the oil is usually low at that point, leaking will stop when the oil settles back into the pan.
You should be able to discover if this is the problem pretty quickly, since the valve and hose are easily visible and also easily replaced. I forgot to mention that the other end of the valve (where it goes into the intake) can also get blocked. a good mechanic can easily check this out in a few minutes if you are not sure how to do it.