Oil catch cans and breather tanks look similar and do similar jobs, but they serve different purposes.

Oil Catch Cans

Normally, the PCV (positive crankcase ventilation) valve uses intake vacuum to relieve the pressure inside your crankcase, but that can result in oil mist and other blow-by contaminants building up on the valve and pistons, especially in direct-injection engines. An oil catch can (sometimes called an air/oil separator) is installed between the PCV valve and the intake, where it removes contaminants and allows only clean air to return to the intake manifold. Some have a reservoir that needs to be drained regularly, while others are plumbed to return the oil to the crankcase.

Breather Tanks

In forced induction and racing applications, the build-up of crankcase pressure can be much greater, so a breather tank is designed to vent that excess pressure into the atmosphere. They capture some blow-by with a catch can and air filter, and they may have an oil return. Importantly, breather tanks may not meet local emissions standards.