possibly the x-ring (or quadring seal) is at fault. if fitted correctly there should be no oil coming from any way past the x-ring.
otherwise, like I have with my Rapide, there is oil passing along between the head and the guide. this could be anything form a crack (not much flesh around the guide) to the guide not fully seated in the head.
perhaps a groove drawn when removing the guide due built-up carbon?
my Rapide had the new x-ring seals on the inlet and the original chamfer/sraper on the exhaust, and even then there was 1 guide that was countersunk to avoid collision with the rocker (using Mk1 cams). when measuring the depth of the exhaust guides, that one guide was seated higher by approx 2mm than the other one, hence the need to countersink that guide.
I don't judge the person/mechanic before me who performed it, I never met him/her and probably never will (quite possibly 6ft under, even that's not sure!). its wrong, point.
got me new inlet and exhaust guides with seals and double the nr of seals as a precaution, and the proper tools to remove the locking ring.
what you could does as a means of elimination, is remove the locking ring, check the seal while your at it, apply some silicone gasket around the base of the guide and refit the locking ring. either the stem is now dry and the seal was the culprit, and/or the guide remains dry which points out to a leak past the guide and head.
oversize guides available from M&S.
