2008 280 DA. Twin 5.0 L Sea Core Mercruisers. Kept on outside rack. Noticed slow oil drip from end of port prop. (Part of exhaust vent?). Would that more likely be leak from motor thru the exhaust or a lower unit leak?
Most likely you need to reseal the lower end. I have found that those seals may begin leaking anytime after 500 hours or 12 - 15 years. I think you are due.
