Can't be 100% sure, but this is what I would look at if mine were doing the same.
How much was the head recon ?, were the guides replaced ?, the cossie eats guides big time, seen them shagged totally with well under 40,000 miles, people also reface the valves and this makes the edges super thin, result can be a few burnt edges and burnt out exhaust valves down the line.
A lot think sticking new stem seals on a knackered guide will sort the oil problem, it will for all of ten minutes.
tabetha