Thanks all for the responses.

I was asking as I have a breather / oil fill cap on one valve cover, and a poppet valve (I believe) on the other cover, from which I have just run a hose to the back firewall. The stock Blueprint Engines setup.

What I find is that I have of oil vapor exiting the breather, with over time puts a nice gloss on my valve cover and results in a drip below the bell housing. I am guessing that manifold vacuum, i.e.: connecting that hose to the induction system would get things flowing the right way. The vent allows air in, the valve allows oily air out, and this gets processed on the wrong side of piston rings.

I don't have any manifold ports available on the Inglese EFI 8-stack. Looks like I may just let the beast mark it's spot ...