Mobil1 Synthetic causing oil leaks is a myth due to issues they had when it was first introduced a long time ago. Since they have added what ever to the product to take care of seals. Still I really question the value of going to synthetic on a old car. I use synthetic in my Talon Turbo but probally not the recent MB 300E I bought with 125K on it. Some advantages one may experience by using it in an older car/MB is the oil staying clean longer, so you may be able to extend oil service. Note from my experience that depends on the car and I have no experience with that in a MB. The other would be if you lived in an extreme cold climate you could use Mobil 1 0W40 all year round and get easy cranking during -20F to -40F(C) below zero and protection during the summer.

Other wise it probally not worth the extra cost if you change the Dino on a regular basis.