When changing the spark plugs in your older Honda, you may find that either the plug tops, the plug threads, or both have oil on them. The two most likely causes of this are (1) failed valve cover spark plug tube gaskets; and/or (2) failed O-rings where the cylinder head meets the camshaft holders (also known as "camshaft bearing caps"). The first is relatively easy to fix. The second is more involved.

The relevant gaskets and O-rings on a 1991 Civic are as follows:

Not all Hondas have the O-rings between the cylinder head and camshaft holder. To see if yours does, check OEM parts sites like bkhondaparts.com (under "engine, cylinder head") for your specific make of Honda, or look down the spark plug tubes and see if they are there. 

Also consider replacing the valve cover gasket (part #2 in the first picture above) during repair of either (1) or (2).

To repair (1), follow your manual's directions for removing the valve cover, adding a step for popping out the spark plug tube gaskets, etc. They will come out easily. Change these gaskets maybe every 100k miles.
 
To repair (2), you will have to remove the rocker arm/shaft assembly. This procedure includes backing off the adjusting screws for valve lash. Thus you may want to wait until your Honda's next valve lash inspection is needed. You can mop out your Honda's spark plug tubes once a month or so meanwhile, assuming they are not filling too rapidly with oil, to no detrimental effect, according to Honda newsgroup regulars. I estimate the O-rings on my 1991 Civic began to leak after about 150k miles and 12 years. On the other hand, "Jason" reported at the newsgroup in 2005 that his 1993 Accord's O-rings failed catastrophically after only about four years and 85k miles. His Accord's engine almost stopped running, and the Check Engine Light lit.
 
For your year and model of Honda, use the sections of a Chilton's manual titled "Rocker Arms/Shafts Removal Installation" and "Valve Lash." These are also available free online at www.autozone.com. From the drawing above, once you remove the rocker arm/shaft assembly, the O-rings will be obvious. I used the manual along with the following for my 1991 Civic.

Disconnect the negative battery lead.

Block the car's back wheels. Put the front driver's side on a jackstand and remove the corresponding wheel, for access to the crankshaft pulley bolt. Put the car in neutral, to facilitate turning the crankshaft.

Partially remove the air intake duct so that the one piping connection to the valve cover is freed.

Remove the valve cover.

Set the crankshaft so #1 Cylinder is at TDC of its compression stroke. Use the white timing mark on the crankshaft pulley and a long screwdriver or other thin rod in #1 spark plug hole to locate and feel for the piston at TDC. I also watched #1 cylinder's intake valve springs operate over a few revolutions of the crankshaft to identify when intake, and subsequently compression, were taking place.

Loosen the lock nuts on all valves. Chilton's says to back off the valve lash adjusting screws "two turns at a time, in a criss-cross pattern" to prevent damage, because the springs of the assembly are still loaded, and tolerances are close. Ultimately back off the adjusting screws at least until they become difficult to turn. Regular honda newsgroup poster Eric said to back them off until flush with the rocker arm, but I did not like the resistance I was feeling, and so I just made a large gap. Before messing with the valve lash adjusting screws, I took some quick measurements of the lash ( = air gap between rocker arm and valve stem). The valves had never had their lash adjusted before, so I wanted to have a feel for how my gage would read.

Loosen the 16 camshaft holder bolts in the opposite order that one is supposed to tighten them. (See the Chilton's manual.) Loosen them about a half turn at a time, so the camshaft is not distorted by the force of the assembly's springs. (This may have been overkill on my part.)

The toughest part of the job may be freeing up the rocker arm/shaft assembly. It does not just lift off. The old oil and perhaps the seals you are trying to replace are baked into place after many years and miles, gluing the assembly down. You will want to tap and pry a bit as follows. Use a rubber mallet for the tapping, because the metal used under the valve cover is very soft. Tap, do not bang, because the camshaft lobes are mated against some of the lobes on the rocker arms, and tolerances are close. Tap in a lengthwise direction. A 12-inch crowbar or similar is fine for prying. Pry only upwards. Tabs are on the outer corners of the two end camshaft holders. These tabs are a good place to pry gently. When I was removing the assembly, the crowbar ultimately freed one end for me. Then I found some more places to pry, and the assembly came off pretty easily.
 
Removing the top half of the timing belt cover (two small bolts) might give you a bit more space to pry and so free the rocker arm/shaft assembly.

Once the assembly is freed up, maneuvering it out is not hard. Keep all the camshaft holder bolts in place on the assembly, to hold it together. Walk it over to a bench. Do not turn the assembly sideways or upside down. If you do, the bolts may fall out, and the assembly will then come apart, whence I suspect life as you know it will end for a few days.


 
The old O-rings peel out readily. I cleaned off a bit of old, baked-on oil here and there, then lubed up the new O-rings a bit and put them in place. Return the rocker arm/shaft assembly to its place. Torque down the camshaft holder bolts per the sequence in the Chilton's manual, to evenly load the camshaft.
 
Set the valve lash per the manual's directions. Just ensure that a feeler gage for the upper limit does not go into the gap but the feeler gage for the lower limit does. I heard a bit of ticking after my first effort, indicating I would perhaps set some of the gaps too wide, so I checked the valve lash a second time within a few days. The second time around I found that the procedure went faster if I set all the intake valves first (rotating the crankshaft as needed) and then set the exhaust valves. Remember that the lobe of the camshaft need only be completely off the rocker arm end when setting each valve. In other words, the piston need not be right at TDC of the compression stroke to ensure the correct valve position for setting the lash.
 
Put the valve cover back on. Torque evenly on the valve cover nuts. Using a torque wrench is by far preferable. Re-connect the air intake housing.

The Usenet honda newsgroup archives have some chatter about having to de-tension the timing belt to remove the camshaft holders. On the other hand, the Chilton manual's steps for removing the rocker arm/shaft assembly say nothing about this. Chilton manuals are not perfect, so I prepared to loosen the tensioner adjusting bolt very slightly and so detension the timing belt. Getting the rubber blanking plug off to access this bolt was not looking easy. Fortunately and ultimately, I found doing so was not necessary. I did jiggle the crankshaft a little (five degrees?) clockwise when I was trying to free up the rocker arm assembly, thinking the timing belt was holding the camshaft lobes fixed, and so it was making it harder to free up the rocker arm assembly. I thought turning the crankshaft a little clockwise would tend to detension the timing belt. I do not know if this was really necessary.

Altogether this took about seven hours. I went very slowly and took breaks whenever I felt tired or a bit annoyed (really, only when trying to free up the rocker arm/shaft assembly). Next time I figure I will finish in about half the time.
I would call the "camshaft holder spark plug tube O-ring replacement" an advanced beginner or intermediate-level job.
 
Thanks to newsgroup regulars Tegger and Eric for the main steps of this procedure.