I don't think any of those fluids are originally specified as synthetic for your car, with the exception that MB now recommends using synthetic for the pre-96 vehicles.

I don't know if the MB products are guaranteed to be synthetic if you buy them from the dealer and specify they are for your 92 300D. And if they are synthetic, I personally don't think they're significantly better than other synthetics available over the counter. Mobil and Amsoil make great products and they should work well in your car.

Any fluid (motor oil, ATF, diff) you buy for your 92 300D - regular or synthetic - should be okay for your car as long as it's the correct type (motor oil, ATF, diff) and viscosity. It should also mix with whatever you have in your vehicle with no ill effects.

I don't recommend mixing regular and synthetic in the same application... there's no harm done, but what's the point in doing that?

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95 E320 Cabriolet, 169K