I measured my head-to-cyl gap in 3-4 places, it was 0.35mm all around both.
Torqued my heads to right Nm i 3 stages as recommended.

I still don`t understand how/why these leaks occur?!?
Could there be miniscule cracks?

Even Viton over time looses its elasticity and becomes brittle and hard. Some of the additives in oil and coolant are capable of forming layers that are one molecule thick.Over time and repeated cycles of heating and cooling the viton looses its seal and allows these monolayers to seep into these small gaps making them larger and larger. Once the fluid has made it past the sealing area hydraulic action takes over making the leak bigger and bigger. Most times the first indication of a leak is a small accumulation of dirt around the seal.
Here are some pictures of common failures in Viton seals. Hope this helps you understand

Chemical failure
O-ring too big
sealing surfaces not clean
Viton not cured properly
Excess Out gassing
Over torquing
Over heat/repeated heat cycles

Very educating, higgy, thx!

I don't know what to do as I replaced the "o" rings about 10,000 km back and they leaked within a few thousand kilometres. Because it didn't leak until 20,000km one must assume the metal parts have changed shape because new "green' "o" rings didn't fix the problem.

you will need to have the head and cylinder checked with a straight edge. Also everything has to be clean and dry on assembly. Soap and water are the only acceptable cleaning agents for final prep. soap and water and air dry. No carb cleaner, no brake clean, no solvents at all.Good old soap and water, same stuff used to do your dishes.

I too had the oil leak at the heads on my 907. I took the heads off and replaced the o-rings with OEM rings, 5 TIMES!!!!!! I even took the bike to two different dealers and paid good money to them and they couldn't stop the leakage. I then tried o-rings that had the same diameter on the inside and outside but where thicker. That didn't help either. I have 52,000 miles on my bike. I had the engine rebuilt last year and the Ducshop which built all the engines for the Ducati racing team here in the US did the work. They said that there was not that much pressure at these passages. They also said that when they did my engine that they fix the leaks. Well, it still leaks, just no where near as bad. I guess you could say they get "wet". Wipe your had down there and you will have oil on it.
By the way, I never replaced the base gaskets at the bottom of the cylinders a single time and they never leaked.

They put liquid gasket around the Orings, is that what almost cured it?
Did they do anthing more?

I think we all agree. That's the way it is!

Years ago,when I worked for Maserati,we used some very expensive Anaerobic Adhesives and Anaerobic Sealants on every o-ring and most gaskets. Just a very very light coating.
Worked very well when applied properly. Might solve the issue here. Getting the right amount was very tricky but other than that and the expense they are easy to work with as they don't harden
in the presence of air so you have all the time you need to work with them.

I have a 1992 907IE. I have taken the heads off at least 5 times my shelf to fix the oil leak and it never helped. I had 3 different dealers attempt it too, but they couldn't stop the leak. My I make a suggestion.
When you replace the o-rings and are putting the heads back on, you will find that you cannot get a socket on the head nuts. Also, remove the frame bolts that attach to the cylinders before torquing the heads, then replace them after torquing the heads. If you make a wrench like the special tool that Ducati suggests (take a combination wrench, with both an open end and a box end, and weld a socket to the middle so that you can attach a torque wrench) you will still need to find a torque converter table(list) in order to get the proper torque on the head nuts. In other words there are conversion tables that use the distance from the nut to the attachment point of the wrench and the angle the torque wrench is to the tool. Using this conversion table will help you get the proper torque on the head nuts. I think this was always my problem because I don't think I ever got the head nuts to the proper torque using the homemade wrench and a torque wrench at 90 degree angle. With the engine still in the frame, to torque the head bolts you will have to work around the frame so getting the proper torque is not easy.
I don't know where to obtain the conversion tables.
du907
du

When I rebuilt my 906 recently I made up a tool similar to the one for the ST2,then I used the torque
settings for a ST2.

Do you have some leaks anymore?
So, could it be enough to re-torque the leaking head?
Ciao

It's quite an hazard loose&retorque an old gasket!