What oil to use in my transfer case

I have a 1991 Safari with 4.2 Fuel Injected TB 42, with Auto Trans.
I am trying to find out whether to use 80/90 Gear oil or Auto Trans oil in the Transfer Box. The manual says that before Oct 1991 use 80/90 gear oil and after Oct 1991 ATF.
I cannot find anything which tells me exactly when my Safari was manufactured in 1991.
Does anyone know how to find exact date of manufacture or why the change was made?
Did they change the transfer box?
Mine is a Japanese import, called the " face-lift model " and has Auto Hubs.

Cheers

Not sure about imported models but on both my td42 and TB42e there's a small plate just above the passenger side bonnet stop which shows the build date, as shown in the pic my td42 was built December 1992

Attachment 50071

Because you have the auto hubs, you will need to run the ATF. The high shear strength of the gear oil would inadvertently cause the front prop to spin, even in 2wd causing the hubs to lock.