Hi Prelude
I had a thought if you havnt changed or added oil to the 28 I think the reason for gearbox oil gain will be the oil seal in the bottom of the oil pump it allows the 2 stroke oil past and it contaminates the gear oil,
The seal often falls out of its recess, I have had this happen a couple of times,I have pressed it back in and gently dented the alli around the seal with a sharp centre punch to hold it firmly in place,
I am sure this will be the culprit,
All the best,
jonmac
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John