There are companies selling *Genuine* Cosworth exhaust valve stem oil seals that are not genuine at all! To the naked eye they look the similar, but you can see the body is slightly different in shape, and when you take a pic you can see the seal is a different colour which is difficult to see in normal light. The one on the left is a proper genuine Cosworth item, the one on the right isn’t, but was sold as one!
The hole in the middle where the valve goes through is slightly bigger on the one on the right, the exhaust valve itself has a step near the top where it goes from 8mm to 7mm, the proper stem seal closes around both sizes, the one on the right doesn't and stays at 8mm, which would let oil through.
May not be what is causing your issue but it certainly would be a valid reason.
all my parts were bought from mat lewis.