Pesky oil leak - is it an imperceptibly slow leak though the quick drain??

[DXB]

The engine: Lycoming O-360 in a '68 M20C. Running well, happily blowing 1qt/4hrs, and the oil turns black fast, but consumption been stable recently with no clear single offending cylinder, so I feel no rush to do anything. 

The history: Three years ago my A&P did the Loc-tite +  vacuum trick on my leaking lower case seam, and it seemed to work very well.  At the same time I had him change my 17 year old Curtis quick drain valve, which always seemed to have a drop of oil hanging off of it, replacing it with one of the SAF-AIR ones:                              https://www.aircraftspruce.com/pages/ep/oilsystems_oildrain/safOilDrainValves_07-00879.php   

The new problem: Increasing oil leakage over the last year. After it sits several days, there's enough oil leak to drip a fair amount on my front tire. Last time that happened, a nut was about to fall off a mag, so I take the new leakage pretty seriously.  On exam of engine, the accessory case and fire wall are bone dry.  The case seam looks pretty dry except at the very front. In that area, there's also oil on the Powerflow heat exchanger, the base of jug #1, the oil line to the prop hub, the back of the alternator, and on top of the air box. There's a bit of oil at the base of jug #2 and also around the top seam for the oil sump and on the bottom of the oil sump.  The O-ring seals on the pushrod tubes seem dry along with the rest of the top of the engine and around the front crank shaft seal.  

My amateurish diagnosis attempt:  I clean everything up, add florescent dye to the oil, and ground run it- I then see nothing leaking directly or by UV illumination.  I then go fly a couple laps in the pattern.  Again it's bone dry when I get back, including the quick drain valve. Figuring I should get the non-FAA blessed dye  out of the oil (which has 25 hrs on it and is black), I do a quick oil change without changing the filter, not spilling a drop in the process.  I still wipe everything down again.  I then come back two weeks later to fly, and there's now CLEAN oil in my bottom cowl and on my front tire . The rest of the engine still looks dry.  

My question:  Could this all be from an imperceptibly slow leak out the SAF-AIR quick drain valve?? Any other thoughts??   I'm wishing I stayed with a Curtis valve. 

 

[PT20J]

Get a vacuum hose cap from an auto supply store and put it over the nipple of the quick drain. If that solves it, you'll know. 

[N201MKTurbo]

I have a piece of hose with a bolt in it.'