@Paul_JJ: x2 ^^
get a catch can mate, even if it's a custom one like mine..
it keeps your rocker cover neater/cleaner as the filter can at times let out coughs of oil..
Better if you get an oil catch tank for the breather, with the filter on top of the tank - this way it will keep all the bad stuff inside the tank and the filter won't get very dirty.
I have filter on top of rocker, and not once has it spat out oil
This link suggests filter should not be on top of the catch can.
http://www.toyotagtturbo.com/forums/showthread.php?30562-Oil-catch-can-pics-%28engine-bay%29
You will usually find that you get heart and soul of the residual oil cheaper because they do not have AA mesh / filter inside them if u buy a non-slip rubber neck