Your crankcase is full of oil mist. If that gets into your intake via the PCV system, you'll find yourself burning oil. Not only is it uncool to have blue smoke coming out of your tailpipe, oil is also very low octane so it can mean a loss in power. Mazda knows this, it's why they put an oil separator on the 2004-05 Mazdaspeed. Unfortunately, it's harder to make a good oil separator than you might think and there's a lot of misinformation out there on how to plumb them in correctly.
Moroso knows, though. Their separator - also known as a catch can - has a tight steel mesh to encourage the oil to drop out of suspension, and it collects in the lower half. The separator can easily be drained via a valve on the bottom and it can be disassembled for cleaning. Includes all fittings to install on a Miata.
This kit uses a larger canister than our other one for increased oil capacity - it measures approximately 4" high by 3" diameter. The drain on the bottom adds more height, so keep that in mind for installation. Fits all NA and NB Miatas, including those with ABS.