Let's say you only have a vent on the engine. Eventually, goopy yellow stuff, which I think is detergents and other additives from the
oil, will start to come out the vent due to a little blowby. At this point, if you remove valve cover
oil caps, you will find the underneath of the caps covered with this yellow stuff as well. Ask me how I know? Because that's how I had my engine set up for a while.
Eventually,, I installed a PCV system and ever since then, no more yellow goopy stuff. It gets sucked in via the PCV and is burnt vs slowly building up inside the engine.
There's a lot of different ways to hook up a PCV system. Depends on your manifold, valve covers, carb, carb space, etc. I don't find it a touchy system, so I'm sure you can deviate a bit and still get it to work.