ZF transmission fluid level check

Here's the relevant page from the manual of my ZF gearboxes.
Also mated to MAN engines, but much less powerful (V8/800), so definitely not the same model of yours, though I would expect the oil check procedure to be the same for most if not all ZF g/boxes.
If you're interested in something more specific, you must confirm the ZF model, because nobody can guess it from the engine type alone.

Neither 16L nor 16K are oil types I'm familiar with. I guess the classification is different between our sides of the ocean?
Anyway, attached is another page from my ZF manual, specifying SAE 30 (non-ATF) as generic oil type, plus some more detailed API and ACEA classes.
The page also includes a list of oil names, but that might well be outdated, since the manual is by now 20+ years old. The specifications still stand, anyway.
FWIW, the oil I always used is the Mobil Delvac 1330.

But again, as I said your ZF gearbox is bound to be completely different (larger) than mine, so while oil recommendations are likely to be the same, I can't be positive about that. I do have some other ZF manuals in my files, but if you want me to check if I've got yours, you should confirm the exact model - just take a pic of the plate attached to the gearbox, if unsure.

Straight 40 weight is preferred, 15w40 is ok to use in cold climates according to the manual. The proper way to check them is to run the engines for 2+ minutes, bump them in gear a few times, shut engines off, wait a few minutes and check the level. I've found if the oil on the stick cold is 1-1.5" above full, then they're good.

According to which manual?
If you check out the ZF manual page I posted, straight 30 is preferred, straight 40 is for hot climates, and multigrade oil is not even mentioned.

I changed the oil in my ZF 665’s after this past season. 30 wt is what they called for.

Same as mine, then.
Out of curiosity, do they call for either yearly or 500 hours replacement also in your manual?
Personally, I don't mind the 500 hours, but yearly replacement sounds pretty much like a CYA driven prescription, IMHO...

If memory serves it as a 1000 her change interval but I would need to verify. Don’t recall any mention of annual. I was doing my first change since they are fairly new.

The OP is asking about a ZF gear behind a 1550hp MAN, but most ZF gears will also use multigrade 15w-40 oil. A few ZF techs and reps have told me if you want to simplify what oil you carry it's perfectly fine to run 15w-40 in them. Because you didn't look on the next page!!!!!!!!!!!! Here's the list for all ZF marine gears. Obviously check which model you have and what's recommended before puting an oil type in there

Bereich
Here is the official link. I like to change gear oil every 2 years regardless of hours, due to moisure possibly getting in the oil.

40w is recommended but it's compatible with 15w-40. I just loaded a 72' with 1300 v12 mans from that same era on a freighter that I've been dealing with for a long time. The 1300 has the same zf gear as the 1550's. I forget the model of the gear and would really have to go digging for it.

FWIW, here's the page of my manual mentioning either annual or 500 hours oil replacement interval.
They do mention also a one-off replacement after the first 50 hours, so good call from your part on that!

I just checked the manual for my ZF 655 gears. It has interval of 500hrs/annually other than the first change at 50 hrs. Regarding viscosity, the manual is silent. There is a full page of approved oils listed by manufacturer and brand name, but no viscosity mentioned. However, the label plate on the gears clearly states Oil Type : SAE 30 and then lists capacity as well. This is why I used SAE 30 when I did my recent change. Interestingly enough, the label plate contradicts the manual as it lists interval at 1000 hrs or annually. That's why I had 1,000 hr. in my brain. The manual is pretty generic and tries to cover a wide range of models. I'll stick with SAE 30 per the label. And I will almost certainly never reach 500 hrs in a year let alone 1000 hrs. But I wouldn't be concerned going over 500 hrs and would defer to the label plate if it was an issue.

Understood. I just find it odd that ZF offers no viscosity/temp chart like virtually everyone else does. I know they keep temperature in mind because they wanted me to put AMOT valves on the cooler circuit for my gears. They were concerned they would operate too cold.