If I recall correctly, if the hose from the oil trap vents to the manifold, you need the PCV valve. In that case the oil filler vents to the open air, preferably with a small filter on top, or to the air filter. This is the normal set-up for most B18s. If the hose from the oil trap is connected to the air filter, you don't need the PCV valve, but should have a flame guard (looks like a piece of rolled up corrugated brass foil) inside the pipe coming out of the oil trap. This set-up is most commenly found on carbed B20s. Of course, these are all air quality control systems, and the first B18s, which came without an oil trap, vented the crankcase through a dump tube along the side of the block and the oil filler cap was open with a bronze wire sponge inside.

I've run a B18 that came with an oil trap with the old style oil filler cap and a dump hose going down to below the engine connected to the oil trap because I didn't have a manifold that had the nipple for the hose with the PCV valve. BTW, I believe the two systems have different nipples on the manifolds, but I don't recall the details of that part of the story.

At the moment I'm running a B20 that came with the oil-trap-to-air-filter hose and no PCV valve, with a PCV valve and that hose connected to the manifold, while the filler cap vents to the air filter. So you can go whatever way you like as long as you put the right components together, and, of course, what your State regulations and/or your environmental sensibilities allow.

Bob S.