So last summer I had a small amount of outdrive lube disappearing from the reservoir and had to top it off a couple times. Could never find a leak outside or inside the boat and no water in the drive. This spring while having the bellows and a lot of other stuff replaced we found the cause. A cracked gear lube fitting. Lube was slowly leaking into the lake.
The real bad news is it is impossible to get to the e clip that holds it in and will require the engine be removed to gain access.

$500 to replace a $10 part. Not a great start to the season.
