The stock Mazdaspeed oil separator is a good idea, but not a great execution. Under hard driving, oil can actually work its way up through the drain tube and enter the intake. This means a lot of oil consumption and an increased risk of detonation as well as carbon buildup in your engine. A lot of Mazdaspeed owners have removed the separator, but now there's a better option.
This check valve goes in the drain line and allows oil to drain back into the pan - as Mazda intended - but prevents it from going the wrong way. The valve has a very light opening pressure to keep the oil from backing up in the drain hose and is designed to work with higher temperatures and oil. Includes all hardware, easy 30 minute installation.