ok here:

the hose you're talking about is called a breather tube, the 420a has a positive crankcase and so combustion gas and oil blows by the rings and into the head, out the breather. stock it blows back into the intake to burn up on the next combustion but it also clogs up your intake manifold and combustion chamber with carbon deposits. putting another breather filter on the end of the line and capping the hole in the intake will prevent this and your engine will work to burn some of that excess carbon out of its system, however with the egr still hooked up you wont prevent this entirely. anyway my point is i dont think it will improve gas mileage persay, but i guess if you have incredible amounts of blowby it could hinder performance and lead to detonation or excessive ecu timing control? sheesh man, i really dont know WHAT your friend was talking about, but then again...neither did he.

get a crankscraper, it bumped my gas mileage up a little bit :)