I live in Mississippi, even south of you. I have always used Castrol GTX 20W50 in my '79 240D since it was new. Since my car is always garaged, I use this grade year-round.
This oil was one of the few approved by Mercedes in 1979.
As to synthetics, I believe that to be largely a matter of personal preference and opinion. While synthetics are superior in some ways, if you change your oil according to the schedule, i.e., 3,000 to 3,500 miles, it is my opinion that the extra money on a synthetic is something of a waste.
All right, guys. I have set myself up as a target. Let's see whose going to take the first shot.