My engine progressively became rather rather oily, and although it's not that easy to be sure why, my bet is it's just because of the CAS O-ring (clearly the leak is rather high and more on the CAS side so I would be surprised if it was not because of it)
Anyway, as it's very cheap (about 0.50€ if I remember correctly) I had not even checked before I ordered an O-ring seal from SJS at the opportunity of another bigger order.
I was very confident until a few days ago when I randomly read here there are actually two seals, one for the CAS and ... one for the other cam shaft too (blanking plug).
As I didn't really want to order just an O-ring from SJS I checked locally, and as always I couldn't find what I needed so went back online to find a French source.
By the way I measured the one form SJS and it's about 32mm OD, 25mm ID (so a 3.5mm diameter section)
Anyway, I found an online source and that lead me to a choice of material for the seal.
They had three sort of seals in stock:
- NBR (=nitrile), shore 70, about 0.50€ each
- NBR (=nitrile), shore 90, about 0.50€ each
- FPM (fluoro rubber), shore 80 about ... 5.8€ each (!)
From what I have gathered on my readings, both NBR and FPM are suitable for engine oil (and NOT EPDM, but there is no EPDM available in that size anyway ^^), but it appears that while NBR/nitrile is good for any application between about -30°C to +100°C, whereas FPM is good for temperatures ranging from about -20°C to as high as at least 200°C (some extra specialized fluoro rubber formulas withstand almost 300°C apparently)
In the meantime I had removed the CAS and back blanking plug from my engine and I do confirm both seals are totally cooked (I couldn't even remove the blanking plug one without tearing it as it was rather brittle, and it certainly became very stiff to the point it looks almost like rigid plastic when pressing it with a fingernail).
Obviously I have no idea how old are the O-rings on my engine, they could very well be 28 years old, so maybe they just need replacing because of their age, but on the other hand I wouldn't be much surprised that it's because of the oil temperature too, which I reckon is at least 100°C (I read that as a rule of thumb it's about 20-30°C higher than the coolant?).
Anyway, I gather NBR is not really recommended for an engine, so I ordered a pair of FPM O-rings.
15€ including shipping for two seals is a lot, but on the other hand cleaning the engine from beneath with "Essence F" and an old toothbrush is not my favourite hobby either ...
(still, I must admit that in addition to a newly painted sump, it's all very tidy and nice to look at now! 
)Any remarks? It's just something I gathered from my readings so I may be wrong
PS: by the way, as for the nitrile shores, apparently 70 would be the default choice, whereas 90 would be intended for high pressure in repeated bursts and that sort of things.
