If you want to follow the "official" route, first look in the Service Manual for the specific lubricant code. The motor oil code for the W123 is 229.1 for instance. Now click on 229.1 on the lubricant page and then you can see the recommended lubricants. You only have to select the desired viscosity.
The manual of the car also has a list of recommended lubricants.
Manual gearboxes of the W123 originally have a ATF type A requirement, but specific gear oils (235.1) are also used (also by MB dealerships), especially on worn-out gearboxes. I have used both and have noticed no difference. Type A is very viscous when cold, newer standards (Dexron II and III) are better in cold weather.
Recommended viscosity for rear differential is SAE 90 or SAE 85W-90. 80W-90 is widely used in W123s, but it is less viscous than it should be. I have yet to see a MB recommendation for 80W-90 for rear differentials.
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