lkchris, What's wrong with regular power steering fluid?

For coolant, MB requires MB coolant. But we know G-05 is the same and works well.
For brake fluid, MB requires MB brake fluid. But we know any DOT4 works well.
For bearing grease, MB requires MB bearing grease. But we know any hi-temp wheel bearing grease with EP additives works well.
For power steering fluid, MB requires MB power steering fluid. Can you tell us why regular ps fluid won't work?

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95 E320 Cabriolet, 169K