I spent over 3 weeks researching the zf lube specs ,but I have better things to do with my brain cells than to sit around and remember specific alloys etc of every little detail after I've finally figured something out, and this is in top of over 30 years of researching various lube specs for assorted pieces of machinery. It would be humanly impossible to remember the last minute detail as to what specific alloy in which part doesn't like what in one oil and another. Some internet bench racers like to start throwing a lot of numbers around to confuse things but when you get the big picture togethr, the research has been done, and I dont' play the throw around the fancy numbers game to bench race and try and out do anyone.. I simply do the research and present the facts that are out there for anyone to find if they want to spend the time, I'm just saving them the time.
Mel is quite correct with the 30 wt non detergent. It works fine, that was an older standby to go thicker than the factory fill of dino ATF which is 20 wt. People seem to forget that these things are not luxury cars, they are trucks that are designed to pull heavy loads and were never supposed to shift like a toyota, especially when cold. The fact that you can put synthetics in them now and get them to shift so easily on a -30 deg morning is just a nice little benefit of progress marching on.
The modern equivelant conversion for the 30 wt motor oil would be to use 30 wt synthetic ATF which does shift better when cold than even the original dino 20 wt ATF, but 30 wt motor oil will shift fine after about a mile or so.
As far as any type of gear oil rendering the zf useless , that is far from true.gear oil is graded differently than motor oil and ATF . As it turns out 30 wt motor oil crosses right over to 85wt gear oil , and 50 wt synthetic atf crosses to 90 wt gear oil . ZF has a lubricant spec sheet that lists ATF, 30 wt non detergent motor oil, and gear oil in a few different multigrades with no extreme pressure additives as all acceptable tested lubes with synthetic ATF being the recomended lube for boxes used with diesels
Any other wt of synthetic ATF or non detergent motor oil or gear oil that falls within those boundries would be acceptable lubricants for that box as per ZF factory specs. THey have come out with a number of different lube spec sheets over the years that appear in different places, but any lube that , in quality is above the grade of the original spec will be considered acceptable.