1991 300SE: Premium or regular gas?

Hi, I have a 1991 300SE and my dad has been filling it up with regular gas for a pretty long time now. The car used to be my grandma's and when she was alive and she always filled it up with premium and would always get into arguments with my dad saying how it has to be filled up with premium rather than regular. My dad, of course, never listened to her and would always reiterate that regular is better and cheaper for the car. Well, now my grandma has passed away over a year ago and now it is officially our car via her will and my dad still continues to fill it up with regular. Now, I recently acquired my license(18 y.o here) and have been learning a lot about cars and Mercedes. My dad is letting my use the Mercedes until I go to college next year and I keep telling him that it should be filled up with premium but he refuses to allow me to do so. He says that the only thing premium does is that it is a little bit cleaner and gives you a little bit more "oomph" on the highway when you are passing cars. I tried to explain to him that while that is the case, it also gives the car better MPG and is faster and more efficient at all times, not just on the highway. He doesn't think so. I am going to show him this page after you guys give some, hopefully good and informed answers on this issue. Thanks!