Modern Vespa

I still believe that, at least in part, reading the oil level contributed to the suspicion that there was/is a problem.

Monitor oil level regularly, ensure correct reading (see above).

If necessary, install oil pan with sight glass.

What vehicle (model, engine, age) are we actually talking about?

The OP has a GTS 250 2007. It's a quasar engine.

#5 is a reed valve for the secondary air supply. The crankcase breather in my Honda free flows into the air filter box with a splash plate in front of it. Blow by thru Valve guide/piston rings creates an oil mist that bypasses the splash plate/ air filter and enters the intake.
Check your spark plug. If it shows it running lean, could be the O2 sensor reading an oil burn too light to see. If you have oil in the air box, this is where I'd start.

Finally, a pic of the spark plug


Looks good to me?

that plug looks terribly oil fouled.

This means the oil went out the exhaust?
I did not notice oil smoke in the exhaust.
In the last 1000 miles I have not needed to add any oil?
I'm not saying you are wrong, just that I am confused.
Also, the engine performs as good as ever.
36000 miles on this fine machine
I do appreciate your comments, Greasy

well, now we are all confused.

here's the deal, the oil has to go somewhere. it's getting pushed into the air box, it's leaking on the ground, it's in coolant or it's in the cylinder and getting burned or pushed into the exhaust.

the picture of the plug suggests that it's in the cylinder and being burned. how much of it, I don't know. but I do know a fair amount of unburnt oil can sit in a muffler.

to me, the real head scratcher is two fold: motors don't just lose a bunch of oil and then stop. they're not self healing. they don't recover, they just keep losing oil. with the noted caveat that if you overfill them, they will "lose" that bunch of oil somewhere and usually be okay. the second part of conundrum is that you're burning oil but not laying down a James bond smoke screen. I don't know, maybe you're just not noticing it while riding? maybe the cat in the exhaust is catching it all and it's just chilling out in there? maybe you were pushing oil and it wasn't getting hot enough to burn off? who's to say.

so, we're back to square one: how much oil did it *actually* lose, how much was *actually* put in and if the levels were *actually* read correctly.

something, somewhere doesn't add up. so I'd be retracing steps, and checking things. find the oil and you'll have your answer.

can you get of picture of the plug's center electrode, ground straps and insulator? basically "head on" if you will.

oh, and we have a saying, they always run the best right before they give up the ghost...

I know my Honda isn't a Vespa.
At 8,000 miles I was getting oil in the air filter box. Online research indicated piston ring blow by. < $150 Replaced piston/cylinder. No change.
Next, I replaced the head and it fixed it. The bike still looses about 1 oz. per 200 miles but the spark plug looks like the bike is running lean. I don't have leaks(but the engine gets oily) and it doesn't show exhaust smoke. I still have the piston/cylinder with 8,000 miles I can drop in any time.
I guess this is the long way to say your plug doesn't look like mine.

A couple more pics of that spark plug.


Spark plug from the GTS 250 that was losing oil

Again

If it takes AI to bed in piston rings, then yes.

With respect, my MP3 lost a ton of oil (into the airbox; I found it) until it didn't.
Engine healed thyself, as it were. It just stopped, and the only intervention I made was cleaning the air filter.

if early on, it could be attributed to the rings fully seating. if not, then perhaps the riding style or conditions changed? ie less WFO, for instance.

but sometimes things DO happen, it's just rare, is all.

I'd say that plug looks worn and oil fouled.

I agree, is a CR8EIB-10 NGK a compatible plug. Scooter west sent it, I am also adjusting the valves, and replacing the belt. I plan to do more riding this season than last. No cannonball stuff, just enjoying the scenery.
Oh, and replacing the shocks with Bitubo.s

This is the plug I use in my 2008 GTS.

Fast Italian Bikes is in the back room of Hooligans on 96 west of Murfreesboro. Its halfway between the Boro and the Castle. He plays with Ducati's and Guzzi's. Maybe he could help ya out with the rebuild/repair/ oil investigation if necessary. Cant hurt to call.

I gave FIB a call and 'YES' they do work on Vespas!!! and only an hour from me.
Thanks for the tip!
PS: I was really surprised they would answer the phone at 5:55 on Sat. evening.