Kanzaki KH18 Slipping Transmission and Oil Choice

[mitbeaver]

So first off the problem we're having is that our KH18 (3QM30F engine) is slipping out of forward gear at low rpms after running hot. Reverse is fine and there are several other threads covering the topic, however I saw mention from several people asking about the engine oil that was used, as the transmission plate recommends ATF, however many people as well as independent sites recommend "same as engine oil." I also noticed from the Yanmar Help site that all Conical Drive transmissions are using non-ATF oils (SAE30 or engine oil). However, it seems like a huge jump in viscosity to go from ATF to diesel engine oil.

I believe the underlying problem is that the conical drives themselves are worn and likely need replacement at some point. I've seen from multiple threads that rebuilds of the KH18 are successful (and costly...). What I'm hoping to gain insight on is if I change the oil will the greater viscosity prolong the life of the transmission and make it less likely to slip for now? Have other people been using engine oil in similar transmission type? Also, could it be the flex plate or other piece of hardware that I should inspect that could be the culprit for the slipping?

[Chief Engineer]

TRANSMISSION TYPES

I will refresh my memory on rebuilding those and post later or tomorrow in detail

[mitbeaver]

Thanks! We had to change the transmission fluid and went with the engine oil and we instantly felt a difference in the engagement went cold, but we are still experiencing the slip in the clutch when its hot.