ZF350A Transmission

Does this gear call for a straight 30 wt oil?

I should really hope that there is not any unburnt fuel in a ZF gear or something is seriously seriously wrong.

Generally diesel oil turns a dark brown/black after running a few hours on most engine brands, usually within a few minutes of it running. You never do get more than 90% of the oil out of the engine in a yacht, between what is trapped in the coolers, lines, and pan that can't be sucked out. Certain brands of engines, the oil tends to stay very clean with the oil looking like it came out of the bottle for the first 50 hours or so. Usually Yanmar's, Cummins, and Cat's stay very clean. The dark brown/black is caused generally by unburnt hydrocarbons, or poor combustion in the combustion chamber of fuel. I've been told that a multi-grade oil as it ages and breaks down it works it's viscosity towards the lower number (for example 15w-40 will work it's way to a viscosity of 15 weight as it ages). In a gear oil sample, I'm focusing on sodium content (a leaking cooler or could be just from a engine room that has a lot of salt in the air, iron (high iron would be an indication of high moisture in the gear oil causing things to rust), and other wear metals such as copper and zinc. A gear never runs hot enough (or shouldn't) to burn off the moisture that ever so slowly accumulates in the oil from condensation.

Cskip has been running technically what is considered the wrong type of oil (Mobil delvac synthetic 5w-40) and was the primary reason he changed it, but the oil also has 400 hours on it. Conventional 40W oil is generally recommended by ZF in Florida (cold climates 30W).

rcrapps=The ZF filters are all white and have just a small ZF logo and the part number painted on them

The sodium and iron look good for the oils age, were any of the wear metals over 50 ppm?

Saybolt viscosity is not given as a percentage ... the way you posted it read as if the viscosity was 50 odd percent of normal. That 50ish number is the reading in Saybolt Seconds ... the time it takes in seconds for specific volume of oil at a specific temperature to flow through a specific sized orifice.
The figures you provided show an oil slightly more viscous than 40wt at 100*F.

You must be very precise when talking about the readings shown on an oil sample report.

That and how clean (fuel/air ratio, atomization)the motors run on cold startup, normal operation (and operating temps-coolant), and how they're run. 2 strokes such as DD's would dirty the oil quicker than 4 strokes from what I've seen. Some seem to run dirty when cold with unburnt smoke coming out of the exhaust such as CAT 3208's but their oil stays pretty darn clean in color. The amount of blow-by is a large factor as well. Generally oil isn't changed because it has broken down, it is changed because of all of the contaminents in it.

Cskip, sounds like your oil has been doing a good job at protecting the gears. You didn't state how many years you have on it. I prefer to change gear oil every 2 years, personally. It's about $200 between the 5 gallons of oil and the 2 spin on filters on a set of zf's like yours, cheap insurance when a rebuild is $12-16k a gear.

I forget exactly what the manual says. I know the manual says every 1000 hours, but doesn't have how many years if I remember correctly. Which every 1000 hours would be fine if you're accumulating them in a year or two (commercial setting). The guys from ZF North America all have told me they recommend changing it every 2 years.

Great. Is the leak new?