I reckon I am pretty good with mechanical stuff having stripped down, rebuilt, overhauled and tuned numerous car engines and drive trains, and my boat engine. I also thought I had a reasonable understanding of oils and viscosity ratings.

Despite this experience I didn't give any thought to the oil I was putting in my boat gearbox. The manual says use the same oil as in the engine so I changed the gearbox oil with the stuff I put in the engine.

My gearbox worked fine when cold but when it got really warmed up sometimes it wouldn't engage the clutch properly. It didn't slip but seemed to grab for a second or two before it engaged fully.

I thought about the oil I was using and had a talk with a diesel mechanic. The engine oil specified in the manual is SAE 30 for Sydney temperatures. The standard diesel oil I pick up from Whitworths or Auto One has a 10-30 rating. The light rating has benefits when the engine is cold and then thickens up to 30 grade when it gets hot.

As the gearbox oil doesn't get anywhere near as hot as engine oil, the oil in the gearbox probably doesn't get near the 30 rating specified for the gearbox.

The mechanic said that the oil could be a cause/contributor to my gearbox problem. He suggested I change the oil before I did anything else.

I changed the oil today and took it for a run. It was only a relatively short run but it got warm and I couldn't replicate the problems despite repeated attempts. It also sounded better. Both of these outcomes might be my imagination but I am pretty confident the oil has made a difference.

Can anybody add to our knowledge of gearbox oils?