I've lived in Texas all my life except for a few years in the Army. (Hear that y'all?)
I started using multi-grade oil only a few years ago. I ran 30W winter and summer for years, changed it often, and my engines ran for a long time, and the ones I took apart were clean and looked good, no cylinder wall scuffing, or bearings looking bad.
My Grandfather was a rancher near Blanco, which is not far from San Antonio, he used 30W in machinery and road vehicles exclusively, and ran them until they fell apart, which took a long time because he cared for everything well. He never used anything but 30W.
It is rare for the temperature to fall below 20 degrees F. and REALLY rare for it to get below 10. If there was ever any damage that may have been done at 10 or 20 must have been minimal.
All that said as testimony for straight 30W, the multi-grade oils have improved so much, it doesn't seem to make sense to use single weight any more. Regardless of your favorite brand of oil, and I'm certainly not going to open up the oil "brand" can of worms, there is a good multi-grade oil available. Just stay away from the ultra wide spread in winter/summer grade, such as 10W40, or I even saw a post about 5W50.
San Antone is a warm climate, but there are very good multi-grade choices. If it were my luck, I would put in 30W and have a record breaking blizzard come through.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in