If it's not the jet valve it could be the valve guides, not as common now but the exhaust guides used to drop down and allow oil to get in the cylinders. New guides have a lock ring to not allow it to drop.
Also check to be sure the valve seal has not been hit by the retainer.
More common if aftermarket cam and /or higher ratio rockers have been installed. I have seen this on the turbo engines after mods have been done.

Randy