Hello all, newbie here!

Should first start off by telling you I live in Phoenix.. I am looking to change my oil for the first time!

....but unknown to me was the previous oil weight and type in the car previously. I was looking for a good recommendation on the brand and weight to use in my '81 Mercedes 300D? I've heard everything from 15w40C rotella to 20w50. Anyone care to help in sharing opinions would be greatly appreciated!!
Thanks ya'll. ** specifically should I use synthetic or not?