I have recently bought a 17ft day boat which has an MD1B engine and Mono Shift reverse gear.

The engine is fine, starts well every time and pushes the boat along nicely. However, the gearbox has sprung an oli leak, not a big one but enough to coat the deck boards above it and the bilge below in fine coat of oil after a day out at sea. I've rigged up a plastic cowl round the prop shaft connection to catch the spraying oil because that's where it seems to be coming from.

So I should be most grateful for some adice and info: Firstly, I can't find any reference to the Volvo MD1B being fitted with the MS reverse gear. Every reference to the engine, including its workshop manual, quotes only the Volvo RB reduction/reverse gear being fitted. So has my MD1B been modified by having the MS gearbox fitted? Secondly, there happens to be a volvo RB reduction/reverse gear unit for sale locally. Could I just remove the leaking MS gearbox and replace it with the RB gearbox? Lastly, would it help as a temporary measure to slow the leak if I filled the MS gearbox with SEA 80/90 gear oil rather than the recommended SAE 20 engine oil?

Thanks and I look forward to some help with this.