What is the advantage of using the catch can, do they catch much oil from a standard engine?

Basically, the idea behind an oil air separator (catch can) is to condensate the oil vapor from the crankcase going back to the inlet manifold. This makes the inside of the manifold MUCH cleaner but also prevents the fuel octane rating from being reduced (due to the oil). Some diagrams are in the link below but heaps more stuff online.

http://autospeed.com/cms/title_Building-Another-OilAir-Separator/A_1101/article.html]AutoSpeed - Building An(other) Oil/Air Separator

They can catch a fair amount of oil on even a standard engine; it’s for this reason that we recommend running one on a standard engine let alone a modded one.