Thanks Curly:

The engine check light comes on every few days (I reset it). I have a
slight octane knock on acceleration/load in 5th gear. Increase in oil
consumption in the last 20k miles from 2k miles per qt to 1.5k. These
are all new problems. Otherwise there have been no change in MPG or
drive ability (500 miles/week). The car (1998) has 100k miles on it
and I have never touched it save for oil filters and tires. The good
idea extends to replacing the battery and air filter and the fuel
filter before I realized it is more difficult. Spark plugs come next
and I am hoping they aren't frozen in the head. I will also be
scheduling it to replace the timing belt. Since the fuel filter is
difficult to replace I am inclined to ignore it until I have the belt
replaced.

The check engine light bothers me though since it is a regular
occurrence in the last few weeks. This occurred at 30K miles and
they replaced the O2 sensor. My next step may be to try to pull the
code. More likely I'll just take it in for service and the belt. Any
thoughts? PCV valve a possibility?