A K&N true claim to fame is that it is a reduced vacume load on the engine,unless you have a 1000hp engin being stifled will you see any real gains in HP
The funny thing you mention about the oil not supposed to be in the engine ,is that well it is supposed to have oil in their ( but i know what you mean i am just being a sarcastic !Removed! for a second)
Also with the pcv system it actually does reintroduce oil into the intake system ( small amounts over time but none the less oil will be burned through a natural engine process.
A drop in filter as mentioned is of little gains with just as little detremental effects unless grossly misused by over charging it,but as long as you give it a good couple of swings around it will expell the excess oil before it can become a problem.