If your Dad's other trucks had the 4 speeds, then yes they do take the 80-90w gear oil. But the zf trans is a newer design and takes thinner ATF.
I did the Lucky Mod and bolted my old DMF together solid. This is about the worst setup for noise. I tried the straight 30w motor oil overfill, I could not hardly shift it when it was cold. I then put 10w30 motor oil in it, still noisy and still a little bit hard to shift on the coldest days.
I finally went back to the ATF. Shifting came back good in all temps.
You know what I found out helped my noise more than anything? Turning the idle speed up some. And the atf in the trans helped with that also. When I would first start the truck in cold weather, the oil was so thick that it would slow the idle speed down, and I would get the noise bad. When I finally went back to the atf, it's easier to turn the trans when it's cold, the idle speed is higher, no noise.
When it's very cold outside, it will be noisy for the first 5 minutes till the oil warms up and the idle speed comes up. And it's always noisy when you lug it in any gear, but you will never get rid of that.