SD50-4T - Oil

Modern transmission oils (ATF) have additives that will affect the "yellow" metal in the cone. But not all SD50s develops the slipping. I have 2 friends with the SD50 with no problem at all. The drawback is that this oil does not hold very much moisture.

As I have mentioned in other posts, gear oils for gearing  have same interesting characteristics.

Basic Knowledge about Gear Oils

All gear oils created  using a base oil plus a "package" of additives giving the oil special properties. There are only a small number of "package" manufacturers, and is recognised in the world of specialist gearboxes that some packages cause serious problems with certain component materials or configurations.

As a member of public or even a professional marine engineer you will never find our what "package" is included in the oil you buy. It is simply too technical. 

GL-4 oils are entirely suitable for most gearing types in mobile machines which includes recreational boats. Gear types are spur, helical, and bevel gearing. 

GL-5 Oils have a target application also including hypoid and spiral bevel gears which are highly loaded, and a lot of sliding action between gear teeth.  That situation calls for even more specialised packages, trying to reduce the friction in that sliding interface, and that is where the issues can arrive. Some packages are very good at lubricating that area, but in the process may have chemicals which can attack bronze, aluminium, or specialised materials or coatings.  One GL-5 oil might be terrible on some materials but another GL-5 oil may be just fine.

As you can understand,  clutches inside sail-drive s mostly include sliding action during engagement and disengagement  and GL-5 oils are formulated in large part to avoid friction in sliding contact area. That is not what you want in a clutch. Bear in mind some sail drive clutches are "dog" type so that sliding doesn't happen. In most cases you want to avoid GL-5 oils when considering saildrives, and there is not enough load to chose it for the gearing.

Back to GL-4 oils. Some GL-4 oils may have an additive package which don't  like the materials  in the drive, or have a slip modifying characteristic which  cause problems for clutches.  You really cannot know from looking at labels. To add to this confusion, some companies may have different packages in the same nominal oil designation/type in different countries.  Yes, that happens, and also they may change the package they use without changing the oil name/designation.

Examples cases to illustrate what is described above.

Case 1. Bus synchromesh transmission  from new the synchro  baulk rings which had  a special molybdenum coating , where burned out after only 7 or 8,000km. Normally one expects installation/adjustment or driver related causes to be the root cause. The bus operators were using an approved oil type. Finally I sent an oil sample to Germany for testing, and got immediate response that the oil package was not the same supplier as the oil approval. Contacting the oils company they confirmed they changed the package, and had forgotten that any change to oil chemistry required them to go through the approval process  again.

Case 2.  Brand new manual transmission in car, with syncromesh 5 speed, limited slip differential, and spiral bevel gears in a combined unit.  Shell Spirax oil was recommended, but after just 200km the aluminium shift forks were worn away and no gears could be selected. Research found that this oil was fond of eating the aluminium shift forks. Changing to a different brand oil, alsoGL-5,  solved the problem completely because it had a more suitable package.

SUMMARY

A. For saildrives use a GL-4 oil, or mineral type engine oil if recommended by Yanmar. There is nothing in the design of saildrives needing GL-5!

B. Find out which specific  oils are working in your country, in the same model of saildrive without causing clutch issues. That is to avoid the oil company switch-a-roo of package issues. Maritime engineers can probably help you with information

C. Don't overfill your saildrive

D. Do change the oil annually, and look at what comes out. If there are signs of water ingress you probably need to change leg seals etc. 

I have SD60 in my H400e, and use a particular engine oil (mineral) available in Australia, which has also passed a particular ZF approval which includes transmissions using multi-plate clutches. It also is suitable for the Yanmar 3JH4e. So far, so good. 

Caveat...what I have written above is my opinion only, based on what I have learned through my professional and person life experiences. I am not promoting any particular brands.