I ran acrosss an interesting situation having to do with engine "venting" recently on a dyno test of a new TCR motor, but it is equally applicable to any motor.
THe only way for oil to drain back from the rocker arm assembly to the oil pan is to flow down the push rod holes. In the case of the TCR motor because of the unique alignment of the pushrods it means the holes are a tight fit. Now is you ONLY have a vent on the valve cover, then all of the venting from the block must come UP the same push rod holes. Now the oil draining down is at 1 atmosphere (14.7 PSI). If the rocker area has a good deal of oil in it (in a TCR this is the case) then all of the push rod holes may be completely filled with oil. In order for the oil to drain down, it must be able to displace some of the air in the block !! If air cannot be displaced, then it must percolate up through the oil trying to run the opposite direction. On the TCR motor this worked very poorly as I could see the oil level in the breather tube rising (meaning the valve cover was full of oil) as the RPMs cam up. As soon as I put a vent on the block, this problem disappeared.
Now once you have a good vent on the block and valve cover, the two will balance each other out. You run those to a three-tube trident mounted higher than the motor, with the third tube going to a catch can. Make sure that the catch can has a vent on it though. I had one customer who had done it all correct EXCEPT the vent on the catch can. This meant the the engine still pressurized itself and the motor and oil was forced out from under the valve cover and the front and rear seals. Once he put a few vent holes in the top of the catch can the engine compartment remained beautifully clean.
The Fiat blocks with the exception of the 903, 1050 motors have a good place for the vent where the fuel pump went. You can use an A12 chain cover on these and put the vent where the fuel pump was mounted. TCR motors using the A112A1000 (58HP) block pose a special challenge. I have not come up with a good solution yet.