A follow-up thought - #6 is typically the detonation cylinder. Where head gaskets blow and rings break. Mis-set timing, low octane fuel, overheating; could be the cause of the problem. Doesn't help fix it but might help focus. Unless oil splashed up on to the valve seats, which seems unlikely at low cranking RPM, the oil in the cylinder raising pressure still points to rings.
If it were my problem, I'd use a borescope to check for cylinder damage first. If there is none, and it runs well, and you get lash set right, take it out for an "Italian tune-up". Nothing to lose. Sometimes people baby their new motors when they should be using them to break them in right.