Hurth kbw10 gearbox noise advice needed

[bony]

Hello,
The Hurth KBW10 gearbox on my Yanmar 2qm15 is making a whining noise in forward gear. It does not make the noise in reverse.

When I first bought the boat the old oil in the gearbox was a light grey colour which suggests it was contaminated with water at some time. I flushed the box out several times with a mixture of auto fluid and kerosene and refilled/emptied several times with clean Dexron 111 fluid.

The ID plate on the side of my gearbox is very faded but I can just make out that is says use ATA-A. I've read the many posts regarding the confusion over which type of oil to use in this model gearbox and I opted to use Dexron 111 auto fluid which I think complies with the manufacturer's ATA-A fluid recommendation.

Some of the posts said that SAE 20-30 engine oil is also recommended and I am wondering if the whining noise I am getting is a result of the Dexron 111 being to thin for the job. Would SAE 20-30 oil help reduce the whining noise problem or would it be a bad move to use it in place of the auto fluid?

The Hurth lubrication topic has been discussed many times on the web but I would appreciate your advice regarding my particular case.

bony.