The valves are made on the same machines to the same dimensions, so stems are the same size, as they have recut the seats(?) the mix up is not really important as they all have new seats etc to wear to.
So long as they have not forgotten the stem seals, it will not be coming from here.
Look in the inlet pipe between intercooler and throttle housing to see if there is any oil there.
Also check your PCV are clear, turbo's can sudenly go but could also be a bit of residual oil in ports used to lubs valves burning off though should stop very quickly.
There is no need to do 1000 mile oil changes at all, btw.
tabetha