I asked the same question while I was at my local performance shop yesterday... Their answer was the same as already stated, not a good idea when you have a functioning PCV system, in fact they said it could also throw a code... Further more, in Ontario Canada if you are pulled over by MOT and they inspect your vehicle, you could pay a $400.00 fine for disabling an emmissions system... They look great, however the consequences out-way the beneifits IMO...

 

You can find aftermarket oil caps that DON'T vent to the atmosphere, if you just want something for looks.

 

Yeah I've seen a few good ones... Thanks,