[Kevin in NJ]

There are 2 points that need to be understood.

600W was a type of oil and not the viscosity.

600W and the modern equivalents carefully research by knowledgeable A people was a heavy steam oil. It is designed for heavy pressure points like the straight cut gears in the A so it does not squish out and not lubricate the gear.
The oils people are advocating using are designed for a shearing action found on the hypoid cut gears. So you may find premature wear from improper lubrication.

If you do not believe me, go read about the oils. I had found manufacturer documentation at one time for both types of oil clearly expressing the type of action the oil was designed to lubricate.

There are secondary properties of the correct oil that you may not have considered. It coats the inside of the metal housings so they do not rust with condensate. The U joint may count on the oils for lubrication and how it flows. Of course, shifting action and gear action change depending on the oil.

Disclaimer: The above is about the pure usage of the oils. In reality most A's are not likely to be driven enough to matter what oil you use. I try to relate the correct information as best known. It is your car do with it as you want.