Thank you all! There is a lot of insight here.
@VOLVO_V70: my mileage numbers are, as always, estimates. I can tell you that since owning this car, I have made 331 fuel transactions for 3,267.059 gallons (if you can trust pump accuracies) and driven 136517 miles (if you can trust odometer accuracy) (as of Thursday). Through 2012 my ownership average mileage was ~42.1 m/g, now my ownership average mileage (before and after 2013) is ~41.8 m/g, which makes me sad. The worst part is, this data is not in a spreadsheet. It’s in a little book.
@Bing, I’d be afraid I’d start a leak if I remove the valve cover. Wouldn’t I have to start all over with a new gasket? I once changed the valve cover gasket on a 1974 Ford Maverick to address a leak, and my incompetence made it worse. I just had the valve cover gasket replaced in May, I would think the mechanic would have said something if he had observed anything out of sorts while the cover was off.
@NYBo, I had never considered that I had installed a different-sized filter. I didn’t consciously choose to do so. I didn’t notice a size difference at that time (Feb 2011). If so, all the filters since then likely have been consistently the same size. My next filter will be a Honda OEM, and I’ll be sure to take note.
@the_same-mountanbik: plugs are scheduled every 105K miles, and I installed platinum Densos at 105,328 miles. I should have another 20K or so on these.
@db4690, thanks, good advice.
@Mustangman. There was a sudden mileage drop (~ 7%) when I bought new tires in July 2013. Michelin Defenders, replacing Michelin X Radial DTs. Both of those are supposed to be low rolling resistance, but the new ones are heavier. I had hoped mileage would come back up as they wear. By now they’ve got 50K miles on them. Michelin had discontinued the X Radial, which I greatly preferred.
I’ve been wondering when the CV joints will go. (The clutch is original, too.) No indication yet (unless this is it). No obvious alternator issues yet. I’ve changed the MT oil twice (most recently May) since I’ve had it.
@cwatkin: I speculated compression but haven’t checked yet. (BTW, Last time I did a compression check on anything, it was in 2002 on a 3 cyl 1993 Geo Metro LSI convertable; it was like driving a big go-kart). Without a code, I had not considered O2 sensors.
Thank you Tester. I’m only under the car 2-3 times a year, but I 've never noticed any deformation of the oil pan.
All you guys are great, thanks again!