I have a 1966 172G with the O-300, and I have the same leaks with about the same usage of oil. Mine has about 1300 hrs on it. I can't offer any suggestions, I'm just in the same boat.

The O-300 (C-145) engines pushrod tubes are press-fit into the cylinder heads.  Vibration and incorrect handling can loosen them.  El Reno sells a tool which can be used to tighten that tube for about $40.  The method requires the rockerbox cover and valve rockers and pushrods be removed.  The tool is inserted in the upper end of the tube and  the tool is then turned to re-seat the tube within the cylinder head.  It takes approx 20-30 minutes for each cylinder to do this job.
The oil consumption you describe is normal.  Continental allows 1 qt per hour as a maximum oil consumption.

Hey David,

I have a O-300-A in my '59 172, and I use about 1 qt. every 10 hours. I am always chasing leaks, if that makes you feel any better...I make a habit of always wiping down the inside of the cowling after every flight. This way, I can detect any new, or significant leaks.

Continental O-300's are great leakers! I put a quart of oil in mine about every 8 hrs. and did a lot of wiping!
Glenn

I have a 1967 C172H and it used to leak out the
bottom of the cowling onto the front tire.  About
the same usage as you are stating.

At last annual, had all the valve cover gaskets repleaced.
Real cheap at annual time and no oil leak anymore.

I have around 1400 hours on the 300-D.

The capillary for the oil temp gauge on most Continentals is held into the oil screen with a brass nut.  Overtightening this nut ...("Hey! Lookie heeeyer, Rufus!  This thang's hardly tite att-all!  Gimme thet BIG wreanch!")... can result in fracturing the temperature bulb.  Then on the next flight or so, without warning, ALL your oil will pump overboard and out the bottom of the cowling.  First thing you'll notice is your RPM starts to drop.  By the time you figure out that carb-heat is not going to cure the low RPM you will have cooked your main crankshaft bearings and will start to look for a vacant field.
Moral:  Any leaks in this area warrant removing the temp bulb for inspection, and replacement of the temp gauge/capillary/bulb with new.  Don't overtighten.  Just snug, and then 1/8th of a turn.

A couple oft overlooked "leaks" are the Gen/Alt seal ( in the generator
or alternator.

Tach drive seal.

Cylinder base o-rings.These use a rubber compound that hardens & cracks
in a short period of time.  Retorqueing has no effect as these seals are no longer pliable.


Next time your spark plugs are removed get a light & check for a "wet" cylinder.
A cylinder can have good compression & still be an oil pumper.

You may find 1. 2 or even ALL of the pistons are WET.
Once oil gets to that point there is nowhere to go but out the exhaust.

Not positive, but I think that the mogas doesnt burn quite as hot or clean, and leaves behind some sticky carbon that sometimes causes the valves to stick.  In the past I've seen owners add Marvel Mystery Oil to the fuel and from then on the planes never had a problem.

IMHO - mogas won't make a difference either way on the valves.  If you follow the threads, more people with small TCM's seem to be sticking valves with 100LL than mogas.  The original TCM valve guides were soft and would wear very quickly.  Oil leakage past the guides caused cokeing to build up on the shaft causing the guides to wear even quicker.  The newer TCM guides would stick a valve easily since they were hardened and there wasn't anywhere for the coking on the valve to go. 
Personally, I think it's time to retire old TCM o-200/300 cylinders (with no disrespect to the overhaul shops)  If the guides aren't put in dead-on-straight you get excessive guide wear with the stock cylinders which can lead to sticking - and most of these cylinders floating around are very old leading to cracks.  I put overhauled cylinders on my airplane and got about 500 hours out of them till they started sucking oil past the valve guides.  After pulling the second jug, off came the overhauled cylinders and on went the ECI's. 
I personally (and for customer's airplanes) have been very happy with ECI's replacement o-200/300 cylinders.  They have a rotocoil on the exhaust valve, more cooling fins and seem to seal up very well.  It's nice to see nearly perfect compressions on these with virtually no oil usage (1 qt every 10 hours for me)
Since the demise of ethanol-free mogas - I run 100LL and lean the crap out of the thing except on climbout.  So far - so good.

David,
I saw your post about updating your airplane with a Garmin 430, Transponder, new radio (?), interior etc.  I have also have a C-172 H model, mostly original equipment. 3000 total hours; 1100 since engine overhaul, so it has more than a few hours left in it, hopefully.  Also, its a straight airplane with no damage or flight training history.
I Was wondering what your investment was in each of your major upgrade projects and your rationale for this investment vs finding a more modern airplane with IFR equipment etc already installed.  I'm going through the due diligence process now and the fork in the road (major upgrade as you have done vs acquisition of a Garmin 696 "uber-handheld," remain VFR and look for an airplane upgrade next year) looms mightily. 
Thanks much for any insight
Steve