The Moto-Guzzi Nuovo Falcone VOC

 I had the same problem. I changed the oil 3 times to get rid of the car oil residue, and it stopped slipping. I was lucky.

Huub's engine was low miieage when the valve broke, with no wear or galling on the valve stem. My theory, recently given more weight by the volkswagon experience is thus.
The pushrods are massive. As the engine heats up they are slow to warm [I've noticed tappet noise goes up and then drops back as the engine warms]. As the engine cools down they are the last to cool. Thus the need to run  big clearences. If the tappets are too tight both the valve seat and the valve have flame constantly over their surface. The seat thus can warp or be forced/drop out by expansion and the valve stem gets very hot causing the metal to lose strength. Thus when the valve cools down sufficiently to hit the seat again it can then break. Huub came to the understanding that the clearances were crucial, even with lighter valves and springs. Loads of old hands in Europe say the same. Keep an eye on the valve clearances and the engines will run and run.
 I guess the cam doesn't incorporate quietening ramps [or it may just be the disparate metals neccessatating large clearances]  so the valve does get far more of a hammering than say my M21 whch will happily glow in the dark and still not break a valve with it's soft springs. The volkswagon problem was confined to one engine size wherein there had been a slight change of head design that had an unforeseen negative effect on heat dissapation.

"I discovered significant wear in the valve guides (and I believe that this is a precursor to stuck valves and catastrophic failure, as we never hear of dropped valves on new or recently rebuilt engines)"

Not sure why you've said this banquo since you were in on another conversation in which a forum member  replaced his valves with thin stemmed examples and promptly had the brand new plot drop another valve. Or perhaps you've forgotten? He came to the conclusion that tappets gaps or rather lack of were the cause. One size of  the volkswagon flat four Beetle engine was also known to do this in the very late 60s early 70s,  To rid themselves of the problem Volkswagon changed valve clearances from .004mm to .006mm. NLM used to set their valves to no gap whch seems ok as long as the engines aren't worked hard, but continental advice has always been that the valves are fine as long as the valve clearances are correct. Since these engines have very heavy valve gear and the engine unshrouded by the fuel tank, the tendancy is to run them tight to quieten the valve gear, Big mistake, standard these engines should always sound very tappety, somewhere between an industrial sewng machine and a concrete mixer..... with gaps set at .010 and .0.20mm respectively for inlet and exhaust cold engine I think. Perhaps you can advise?

I would run the  idle at the same speed as a brit single so that the bypass valve opens up and feeds the cambox. Running it low is a party trick

Mine was the one that started smoking when I added the line. You can probably buy one cheap from NLM. I raised the idle slightly and it only smokes now when the oil level is high, which I think is more to do with splash than anything else. Oh and it smokes when on the sidestand, which rather proves the point. Don't run car oil in it and don't follow NLM's advice as to valve clearances. I put raisers on the front seat mountings as standard it's set up for italian dwarves.

I've no idea what the timing is on mine. I just lined up the scratch marks on the points back plate..... However being brought up on manual advance bikes I can feel when the timing is off. What's more significant with these motors are the advance springs. Too slack and they are massively advanced straight off idle and too strong and it won't pick up off idle. Don't ask me how I know..... Oh and run the valves on the slacker of the two recommended settings and pay no attention to North Leicester Motorcyles's recommendations which are not fine if you run the motor hard. If you have  the carb misbehaving after a downpour take the fuel cap apart and fit a tight rubber O ring round the square thread. Due to the Dell Orto float bowl design these motors don't run badly when they get a smidgeon of water in the fuel, they stop.  And mine doesn't start until all the water has collected back in the float bowl and been blown out with compressed air, some hours later....... Don't run the idle really low as the cams only get fed with a bypass valve once the pressure is up. Running it at lowish brit single revs is fine on mine. Don't use car oil as it causes clutch slip. If you do have a bit of that change the oil and run for a while a couple of times to flush out the car additives. I use Morris V twin oil, after all a single is half a V twin.....  North Leicester say Moto Morini clutch springs are identical. They were very helpful.

 I beg to differ. Having turned over the engine by hand with the rocker box covers off, great dollops of oil were being deposited on the ground; this  before  fitting the additional feed which also delivered an excessive amount  of oil by the same hand cranking. I expect the guides are worn, but since it wasn't using any oil, it wasn't a problem. And I don't believe excess guide wear causes valves to break since that was recorded as happening with brand new thin stemmed valves which also showed no sign of galling. Indeed the builder thought it was due to running the sort of  tiny clearances promoted by North Leicester Motorcycles. That may be fine for chuffing around at 45 on a touch of throttle  but not at speed. I think what happens is the massive pushrods expand a lot and hold heat longer than the castings. That holds the valve off it's seat leading it to soften and then break when the engine cools down sufficiently for it to start slamming into the seat again. I've run the engine for miles at 70+ [without realising it]  and taken it up to an indicated 78. The only problem being the ignition keys blew away....
Oh and I got severe leg cramps after around 250 miles of mixed welsh roads so I got the saddle raised at the nose by 4".  Didn't happen again after doing an all day orienteering event  so that worked.