Okay, some more information.

The original rubber seal on the cap is still present. I will be ordering a replacement just in case on my next parts order.

The oil trap had a small amount of gunk in it but is pretty obviously not clogged or in dire need of replacement.

I do run synthetic but I do not know the car's entire history.

I did have a moment of inspiration the other day and removed the nipple that provides vacuum to the flame trap assembly. It was almost completely plugged with oil residue. I cleaned that until it was sparkling clean inside and out and replaced it. The quantity of oil leaking is greatly reduced; I suspect that the rest is actually being caused by the o-ring (I think it's shifted and not sealing any more, leaving a gap between cap and valve cover), so I'll remove that and see what happens.

When you added an o-ring to the oil-cap, was there a flat rubber seal on there already?
Like this? (http://www.fcpgroton.com/product-exec/product_id/11785/nm/Volvo+240+Oil+Filler+Cap+Gasket+(1985-1993))

If you're outright missing that, it would make leaks happen easily. They also get old and hard and stop providing a tight seal.

The oil-cap suck-down test only works on models with the metal cap. It will not work on a plastic cap.

If you're not getting any vacuum at the manifold nipple, and you have a bit of oil going through the throttle body, I would run some seafoam in the intake. It's been called "chinese water torture". You alternate distilled (has to be distilled) water and Seafoam about a teaspoon at a time through a manifold vacuum line. Dipping the line for a half second will be enough. The engine will sound awful. When you're ready, put the line in for longer, and the engine may die or just run really slowly. Let it die or shut it off and wait 10 minutes.
When you restart it, it will blow smoke like a brush fire for about 3 minutes. I recommend doing this somewhere away from people (at least people you care about).

I did it to a 240 I bought, and got a stain about 6 inches around of soot shot out the tailpipe.

I also think that replacing the nipple and the breather box is a good idea - but I strongly recommend buying an OEM box. I think my aftermarket is leaking already.

Good Luck! And it's probably not the end of the world. If you do start ejecting your dipstick, then start worrying.
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1990 740 Turbo, on its way to stock specs, maybe beyond

I would clean or replace the oil trap. They become plugged with carbonized oil after many years of use, and when they get plugged the engine gets high crankcase pressure. It is under the intake manifold, at the back of the engine. There is information in the 700/900 FAQ.
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john