OK, my '00 Civic EX has around 156K miles and runs great. I checked the oil the other day and found that it was a little over a quart low. It had been about a month since it was last checked and topped off. This was the most it had ever been low. I exclusively use Mobil 1 5W-30 synthetic oil and Mobil 1 oil filter. Have ever since I bought the car w/ 59K miles on it. I perform all maintenance as required. I drive at least 400 miles a week, 95% highway/interstate. The car doesn't smoke at all, so I don't think it's burning the oil. There are no signs of leakage in the driveway either, so I'm not sure where it's going. I know this seems like a silly question, but what could be happening here? All suggestions are appreciated. Thanks in advance.

You are burning oil slowly.

That makes the most sense. Now, some new questions. I assume replacing the piston rings would fix this problem. Is it worth it to do this or should I just keep topping the oil off? I also assume that it will progressively get worse? I've heard of others going to a heavier weight oil to help with this. Is this a good idea? I know 5W-30 is the recommended weight for this engine. Is using a full synthetic worthwhile now that I know I am slowly burning it? Or would going to a conventional oil only make it worse? Sorry for all the questions. Just trying to find the best course of action. Thanks again.

If you really want to fix the oil burning problem then it is much more cost effective to replace the engine as a whole, but that's still asking a lot. It would rather be wise to refill the engine with a heavier oil, Mobil 1 10W-40 would be a good choice, and see what kind of improvements you can see. Remember, when the engine burns oil internally, it is mileage, not time, that is the factor.

def sounds like rings..if heavier oil dont work,it might be the beginning of worn rings

try a compression test,then check each plug for deposits

UPDATE:
Ok, my father and I were working on the car this weekend (A/C compressor) and out of curiousity I took a closer look underneath. Upon examination, I noticed oil on the transmission side of the oil pan. I can't tell exactly where it's coming from. I do know it's not coming from the oil filter. I was thinking maybe the oil pan gasket, but my dad suggested it might possibly be the rear main seal? He said either way that the entire engine would have to come out to repair and that a rear main seal replacement is a pain. The "leaking" oil isn't enough to create a spot on my driveway where I park, so this must be a slow leak. Any thoughts on the best course of action? I just hit 166K this morning and would like to keep this thing going for at least a couple more years. I don't know anything about rear main seals, so if someone has experience w/ this, please share any info you have. Thanks.