I also have a 98 civic. If the tank is below 1/2, it
takes extra cranks to start if you don't give the fuel
pump a chance to prime the system. I have seen this on
other Gen 6 civics as well, especially after sitting for
more than 3-4 hours.

For some reason, when the tank is full, it doesn't take as
long. I assume this has to do with the orientation of the
fuel pickup or something, though I have never inspected it.

Perhaps by coincidence, your tank was below 1/2 after the
oil change, and you just happened to notice this due to
DIY paranoia. I been there.... for instance: after my t-
belt job.

t