I had a customer that had the same issue with 2 of his cylinders on his 912 RV-12 I called lockwood and apparently it is an issue with the exhaust valve guide oil seal being made of imperfect rubber(basic description). The seal when working correctly will keep engine oil from the head from making its way down to the face of the exhaust valve....but when it expands or comes off its seat it will allow oil to leak by, I filed the defect paperwork to lockwood and have been waiting to hear back since mid April. But having covid throw a wrench into things I have never heard back. On the positive side after about 50-60 hours the leak has subsided, my guess is the rubber seal finally set against the valve shaft. From everything me and the owner have seen there were no adverse effects on engine performance or maintenance, besides from having to wipe the cowling and heads clean. Not the best answer but hope it helps for understanding why and where the is leaking from.