I believe the Mobil1 75-90 is their standard syn-gear oil. I have switched over to this for my differential about a year ago. I am using this in a 201, not a 124, but all my materials urge against synthetics as well. Like you, I think that may very well be based upon old information.
I talked with a very reputable benz mech a couple months ago. His take was all the Amsoil and Mobil1 synthetics are of good quality and are okay to use. He did strongly urge against using synthetics in my auto transmission though.
I think the issue of causing leaks is not a yes/no issue. If a car has gummed up seals/gaskets from years of using poor quality fluids and infrequent changes, then yes a synthetic will cause leaking when you switch over. Synthetic fluids act as cleansers and with it take the crud that was often plugging the gummed gasket. If the car has always had regular fluid changes and decent fluids, switching to synthetics will only make things smoother and last longer.