Oil in the spark plug recesses is not uncommon for the B234F 16-valve head. My '89 started doing it when it was only a couple of years old. It isn't serious. The valve cover gasket simply isn't sealing properly. You can try re-torquing it to an even 13-14 ft-lbs or even taking off the cover and applying a little sealant (as noted below) to see if that helps. The back pressure from a plugged flame trap will often make such oil leaks all the worse. With the engine running, loosen the oil filler cap and do the old suck down test. If it rattles loosely or there's no apparent suction holding it down then clean/replace the infamous flame trap element. Make this part of your routine service along with inspecting and cleaning the intake throttle body and vacuum hoses -especially the small hose off the flame trap. The B234F is especially noted for collecting crankcase oil vapour.

When the valve cover is off next, which hopefully will only be when you get sick of the oil leak, replace the gasket with an OEM quality one (as I recall it's double ribbed red rubber, but I may be mistaken). Be sure all surfaces are clean. I believe Volvo calls for a drop of blue RTV sealant on the gasket at the corners of the front and rear camshaft journals, but why mess around, especially as your valve cover may have a very slight warp. Use a thin smear of RTV on both sides of the gasket around the spark plug recesses and over the gasket surfaces approaching and over the front and rear cam journals (especially the one for the distributor).

With this large aluminum cast valve cover, to help prevent warpage and to get a nice even gasket compression, I like to torque it. Call me a purist, but I now even do that with the stamped valve covers used on the other red block engines as they're easily warped if they're overtightened and the lip gets crushed (Volvo calls for a smear of blue RTV in the four corners of those valve cover gaskets). The spec for all nuts is 11 ft-lbs (which is slightly over hand tight when using a nut driver). Do it crossways working from the center out. I liked to re-tighten it after a year or so, backing off the nuts and increasing torque by about 2 ft-lbs. That should permanently end the problem.

In the meantime, at every oil change remove the spark plugs, read them to check engine health, clean/gap as needed, wipe out the spark plug recesses with a paper towel and keep the plug insulators and spark plug wire boots clean to avoid baking on traces of oil.

With an oily recess, I also noticed an occasional loose spark plug when I came to do the next oil change. If you notice that then increase the plug torque from the specified 18 ft-lbs to 20 ft-lbs (add another 2 ft-lbs for a used plug) -be sure to start hand threaded and don't get carried away as it is an aluminum head after all.
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Dave -940's, prev 740/240/140/120 Never overestimate what little I know.