Many shops replace the PCV valve at 60,000 mi intervals.  Many folks also
have success with removing the valve and cleaning it with carburetor
cleaner.

Eric

At 140k miles and16 years, replacing either the O2 sensor or
PCV valve or both may improve performance. It's not
guaranteed. OTOH, the parts are cheap enough to warrant
replacement about now.

Did you pinch shut the hose to the PCV valve and wait at
least 30 seconds, listening to the click? It's not terribly
loud, but it is noticeable. With the engine off, you should
also remove its hose, then the valve, and inspect it. When I
did my 91 Civic at about 138k miles, I found the valve top
chock full of black waxy buildup.

I would just replace the PCV valve at this point. Its
springs, for one thing, do wear.

One other benchmark type test: With the engine idling,
remove the hose from the top of the PCV valve, then cover
its top with your finger. You should be able to get the
"click" this way, too, on your 90 Civic.

Best prices for genuine Honda (= OEM) oxygen sensors (IIRC
around $45 for your car): https://www.automedicsupply.com/

Best price for a PCV valve:
http://www.hondaautomotiveparts.com/auto/jsp/mws/PartsSearch.jsp
http://www.slhonda.com Click on "Parts and Service," then
"Buy Parts Online."

With shipping, buying the PCV valve at your local dealer
might be a better deal.

I replaced my 91 Civic's oxygen sensor pre-emptively at 154k
miles. I didn't notice an improvement in performance. On the
other hand, I probably won't ever need another oxygen sensor
for this car and so have avoided a sudden failure of the
oxygen sensor (which admittedly is not devastating; just
annoying).

A few symptoms of a failing oxygen sensor are degraded miles
per gallon and engine running rough. There are voltage tests
one can do on it, too, but I think that's overkill for an
older car.

Only buy OEM for these parts.