Brennie, if you use the search function, you will probably find about a million posts on this forum about this topic. Study those posts, I think it is worth it.
I think there may be more opinions and recommendations than Members, actually.
As a quick answer from a non-mechanic (myself) here are the key takeaways for me:
1. Stay with what manufacturer recommends, which is 20W50. You will end up with mineral oil most likely.
2. What is very important for oils for our cars is the ZDDP content - this is due to the type of timing mechanism we have. Most of oils today have lower content of ZDDP than in the past and not enough to protect our camshafts. It is easier in the US to find an oil with proper content of ZDDP (do not remember the number now) - I am hearing most of the muscle cars use it. There are also additives available. The only brand I could find in Europe, or its part where I live with proper content of ZDDP (and believe me, I spend quite some time on it, including mailing to various companies HQs) is Valvoline VR 1 20W50. This is what I use.
The oils used by Mercedes today do not have the proper viscosity and they do not have the proper amount of ZDDP.
So this is from a non-mechanic with overhauled engine. I am sure you will hear more form other Members.
Please change oil every year, even if you do not drive the car a lot. I buy oil filter change set from Mercedes dealers - this way you get several seals you need in a package.