In these old Panzerwagons, the oil soots up much more quickly than in the modern day super high pressure Bosch type systems as used in the modern Benz, VW TDI, or GM Duramax.
As you are in a Florida climate, the low temperature benefits of a 5W-40 are largely wasted.
In my 84 300SD, I use Mobil Delvac 1300. Probably any of the CI-4 rated 15-40 oils would do a great job for you. I would change oil at 5000 miles or less to keep soot levels reasonable.