Lubricant Oil for the "C" (not z) drive
Ultich and Pat - Here is what Olivier Beaute had to say on the subject 4 years ago.
the C-drive has been designed by AMEL in 1989. There has been 3 different designs for the Santorin and Super Maramu, only one for the 54. Until the last 54, AMEL has recommended to use 20W40 oil only because that's the kind of oil you usually have on board (for engine and generator) if you need to top up the C-drive tank. And that would avoid having two kinds of oil on board. In fact, this C-drive did not need this kind of oil (20W40) as it is made for high temperatures (that you find in a diesel engine). The C-drive oil rarely warms up more than 70°C.
Now, the manufacturer of the C-drive has changed and recommends the 80/90, and probably would deny warranty if this kind of oil is not used.
Again, in order to make things more simple, AMEL decided to use 80/90 gear oil for the new models, but also when they replace the oil in the older C-drives. I have seen Super Maramu's C-drives with SAE 80/90 oil replaced by the AMEL service team in La Rochelle (with the label mentioning the grade of oil and the engine hours attached to the C-drive tank).
So, if you (AMEL owners) replace oil in the C-drive, it's OK with 20W40 or 80/90. Don't mix both oils, but in case you do the of oil service of your C-drive, if you don't find one kind of oil, you may replace it with the other kind.
The 55 and 64 owners will just follow what they are told in their owner's manual (SAE 80/90).