What's the difference and how do I determine what I have? I've seen oil caps for open and closed crankcases. The cap that came with my valve covers has a nipple on it for a vacuum hose. I believe it is intended for a closed crankcase. I have nowhere to connect the nipple in the oil cap. Should I just plug it off? Buy a different kind of oil cap?

 

That nipple is for attachment to the underside of the air cleaner. It's a breather cap. Other breather caps have a filter built in and no nipple. There are some caps with a built in PCV valve that look similar. They would be on the other side of the engine.

 

All crankcases have to inhale and exhale. If you do not set them up to do so through breathers, they will push and pull gaskets. PCV Valves or Positive Crankcase Ventilation Valves systems are designed to vent crankcase gases into the intake to be burned in the cylinders instead of simply being vented as pollution into the air. For plain jane engines, this works well. For race motors or old motors with excessive blow by, PCV systems do not work well as there is a bunch of oil vapor entering the cylinders and can foul plugs. PCV systems also do not move a bunch of air, so at high rpms, it can be insufficient...in my experience.

That being said, with your stock-ish motor, you should have a PCV valve connected to manifold vacuum in the passenger side valve cover and either a filter breather or a breather connected to the air filter via a hose on the driver side.

 

Since I have nowhere to attach a hose, I went to the parts store and bought this: