Anyone know if this valve can just easily be removed and O ring changed or new seal?
It's starting to leak some oil from what appears to be from its base and onto the plastic cam cover.
Jag Classic don't seem to show any details of an O ring or seal for this valve.
Picture of breather valve below:

 

it is old as **** it will always leak

replace the osv they’re peanuts

 

yes its easy to remove and theres some orings you can easily replace

 

2 days ago I removed that same breather (at least it looks the same from the outside, which is all what counts here) from my 2.5L X-Type. I removed my front cam cover (which would be the same as your left cam cover), removed the plate from underneath the cam cover, saw the breather from underneath, and then figured that it is very hard to take it off. Mine also leaked oil. I wanted to remove my cam cover anyway (needed new gaskets), but you actually do not need to remove the cam cover. How to remove that breather CAREFULLY:
if you look VERY close, you will see 2 miniature triangle plastic protrusions on the breather pointing towards the cam cover (180° offset), which claw themselves into 2 impressions in the cam cover. Without those it would be a child's game to use a pair of plumber's pliers and to carefully crab the square shaped section of the breather and to turn it ANTI-CLOCKWISE. But the breather is stuck, because it claws itself into the cam cover impressions, as I wrote above. I am sure that if you apply simply more force you would break the breather. Thus I did what I always do when I want to win a fight with a fragile bit of plastic in the engine compartment: I use my HEAT-gun: I heat up the plastic of the 2 claws very carefully (don not damage/melt the breather or anything else) to make the plastic a bit softer and a bit more flexible and THEN I turn the breather as described above anti-clockwise (a bit, then I heated up the claws a bit more and then I turned the breather further. It "turned out" alright (no pun intended...

And yes, the you get to the seal, which needs replacing. I very much doubt that Jaguar is offering it. I measured it and came to the conclusion that I most probably need a FKM (material) seal ring of the following dimension: CS 2.5mm, OD 20mm (than means: a seal ring with a cross section of 2.5mm, an OD of 20mm and an ID of 15mm). I found it on ALiexpress (cheap) and now I am waiting for it to arrive, while I am still busy fixing the rest of the car.

 

Thanks for your excellent information Peter! I will give it a try.
Just had another look and oil has made its way down the whole width of the plastic black can cover, under the 'Jaguar' embossed black plastic cam cover dust cap and through to the alloy.
Looks worse than it is and it probably started because I've recently changed the leaking cam cover gaskets so the new found pressure had to go somewhere!

 

If you have plastic cam covers, then you have the V8 version of the S-Type. I have V6 S-Types (after 2004 - magnesium cam covers - also a PITA), but I have those plastic cam covers on my V8 X308 (XJ8). I wrote a thread about the incorrect use of plastic parts in the hot environment of the engine compartment. Those plastic cam covers are prone to brake and are very expensive to replace parts. I fixed my slightly cracked plastic cam covers by drilling holes into the plastic and hamming nails into those holes. It sounds worse that it is - if you want to know more about that, then check:

https://www.jaguarforums.com/forum/x...covers-266148/

So yes, I also had oil leaks there. Replacing the cam cover and spark plug seals the "normal" way (i.e. just adding a bit of gray liquid sealant at the dedicated uneven spots (where parts meet, e.g. the timing cover) did not do the trick. I had to remove the cam covers again and be way more generous with red hot temperature silicone gasket maker (to make up for signs of damage of the former shape the plastic cover had where the seal sits). That worked.