Sorry to bring up this "old chestnut " but would welcome some advise.
Both cam covers are bone dry and no traces of oil from any edges.

Pulled up at fuel stop tonight and noticed smoke comimg from drivers side gap between wing /bonnet so assumed the worse

.When checked have a visible oil leak comimg from the 3rd bolt from the front outer edge of o/s cam cover bolt

.No other bolts have any markings and entire cover except where oil has dripped from just this single bolt.Put an E10 socket on it and it wasn.t loose so have undone it ,cleaned up mess and refitted it torqueing it down to 8nm as should be.

Would the "0 " ring have perished,or gone brittle and caused the leak

as its just this 1 area, which I will check again after drive to work in morning.Seems like a lot of work and expense

just to resolve 1 possibly not missing "0" ring or could I just remove the bolt,fit a rubber washer underneath it and then refit the bolt and ...hope for the best
