What is The Best OIL to use in Gasoline MB engines?

Hello All,
This is my first post, but by no means my first time viewing them here. I really, really wanted to thank everyone who posts here. There are millions of us who come here to see your valued and very informative answers. We cannot thank you enough for this forum. Please know that you are very much appreciated !!
Anyway, I recently purchased my first MB, even though it is a 1986 300e, with 166,867 miles on it. I have always dreamed of owning one, I have been reading on them all my life. My family could never afford such a luxury automobile, But I am 37 now, and I am fulfilling my dream. I test drove a few a before I bought this one, and ALL were very, very, solid automobiles. All one has to do is TEST DRIVE one, and it is understandable why they are truly built to last a lifetime. I want to give special thanks to the senior members of this forum, (larry biddle, benzmac, etc..) I wish I could list everyone, you are ALL legends to us and are looked upon as great contributors,you all deserve lifetime of achievement awards !! And if we all could give back half as much as you nice people have,then we would be half fulfilled, LOL! (laughing out loud) believe me, your comments and suggestions will live on forever. here Thanks again..
OK, now, I have searched for this question, and haven't found it before, but I think it has to be one of THE most important (maybe simple, maybe not) questions..
What is the BEST Oil to use in our MB engines ? this can be for both gasoline and diesel. Oil is the lifeblood of an engine, I think we all know about changing it every 3,000 - 7,500 miles is a no brainer. Lets talk about engines (both gas and diesel) that are new or used...what do we start with? synthetic or multigrade?..engines that have 100,000? (and we dont know the history of what the previous owner really used) but what would be BEST recommendation from everyone. How about engines with 150,000 miles? As an example, my previous owner SWEARS by Rotella - T 15w-40 oil. Well, that is one of the Cheapest oils (as far as price) that
there is.!! My engine is strong, and it doesnt leak any oil , but I wonder if I used Castrol or a synthetic would it be more powerful or last even longer?.. I have heard the stories about not putting in synthetics because of this and that (depending on miles on engine), but lets be real, Viscosity breakdown is Viscosity breakdown, and if an OIL is better, cannot it not be OK to change brands for the better? (meaning better for the engine parts to be protected with the BEST possible protection for viscosity protection, carbon buildup, etc? ..) I look forward to this lengthy discussion, and hopefully this post will be a favorite thanks