2004 subaru forester oil in spark plug wells

Highly unlikely. It's probably the tube seals (which is usually part of the valve cover gasket set) but maybe a Subbie guy will know better

The head gasket could be bad but it won't cause the oil in the spark plug tubes.

You're both correct. Here's a possible cheap way out: remove all four plugs and wipe them clean, as well the cable boots. Replace the plugs with iridium ones so that you never have to again, while you're at it. Replace oil with 10w40 (or 20w50 if hot climate), and add a bottle of stop-leak conditioner. Wait a few weeks and see if the problem has abated due to gentle seal expansion. If not, you can replace the plug gaskets. A HEAD GASKET leak generally looks different, in that seepage emanates from the lower inner edge of the HG instead of just the bottom of the valve cover. The HG OIL leak usually starts on the LEFT (pass) side, whereas the HG COOLANT leak starts on the RIGHT (driver) side, at #4 cylinder rear corner. So try the thicker oil and chemical fix first, as it's super-cheap.