I don't know how long you've lived in Dallas, but "never gets below 40" is not correct. We've had warm winters for the last two years, but it has gotten below 40 many times in the last two winters.
For determining oil weight, I would plan on the low twenties, keeping in mind that it can and has gotten into single digits.
My suggestion to you, would be to get out your owners manual and find the oil viscosity chart. It will give you oil weight versus temperature range information you need. Even if you don't drive very many miles, change it in the fall and spring minimum. You could change to a different weight for the winter and back to your summer weight in the spring.
It's just me, but I would get the factory oil out now, even if you are planning on longer than 3.100 mile oil change intervals. Any microscopic particulate matter that may be in it from the first few miles, you can get out by changing it now.
Change oil hot and change oil often,
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in