Great, you have admitted a zero net pressure differential from piston movment, AND a postive pressure differential from blow-by ( with a disclaimer that if the PCV cannot handle it, then you need a rebuild )

SO.. why do you say run a breather cap, if the OEM calls for a sealed cap?

This is a very relevant question to me. I have a 64 C 10 with a 350 which has little miles on it, and has leakage at the front seal. It is equipped with a K&N oil filter breather. it has occured to me that if I were to put on a sealed cap, the front seal may very well stop leaking. ALSO.. I have a 78 firebird with a 305, with MANY miles on it. Taken it three times from GA to CA, plus about 300 miles p/wk. it has one of those cheapo chome breather caps with fiber filter inside. it is showing definite signs that rings have exessive blow-by: oil film accumulating outside the rocker am cover. but then, that engine has had it rough. Overheated ( severely ) and I don't hesitate to flog it to smack a ricer.

what is my point? dunno.. been drinking... working day is done..

anyway.. I would like some better information to decide to keep the breather cap on the low-miles 350 or not. i think it is pretty evident that the firebird is better with the breather cap. Although the environment would be happier with a sealed cap.