A question, this time have any of you guys (or girls) suffered from a plug lead working it self off the plug?

Finding No 4 plug lead seems to work it self off the plug gradually. New genuine Subaru leads at the last major service about 10,000 ago.

Len at Subaru4you who services it did say the inner rocker seal gasket around that plug was leaking a bit allowing oil to seap into the gallery/holes where the plug screws.
Not seeing any oil in there now but plug lead seems to work looser. Wondering if pressurised air from the rocker cover is finding its way past the plug lead seals down to the area around the plug and gradually creating pneumatic pressure working the lead off bit by bit.

It's a MY99 non-turbo engine, but the turbo engine I had in another car had similar rocker seals around the plugs when I replaced them