I've heard some say they are good and some say no good confused
I tried a Moroso set up one summer, the only thing I saw was more moisture in the valve covers when I removed them to check the lash down
Vacuum in the crank case can help stop tiny leaks and make power above 6000 RPM on a several good engine dyno tests that I know of up
The more crankcase vacuum the better the power gains above 6000 RPM work

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)