I also live in the Dallas area and drive 60k to 70k miles per year. I use 15W40 in all my cars, and I have driven them to high mileage with minimal problems (after all no machine can last forever) My 240D went 516,000 miles on 15W40 before finally giving up. My 300E has had nothing but 15W40, but it's made it only 168,000 but still in great condition.
I read many years ago that 10W40 was the one to stay away from. It seems that there is an additive called VI. This is the additive the allows the viscosity index to stretch from the thinnest to the heaviest with change in temperature. 10W40 has an extraordinarily high amount of VI. The high amount of VI additive causes extreme carboning problems. I have never experienced this with 15W40. It appears that it takes a very much higher amount of VI to make the stretch down to 10, than it does to make it down to 15.
As for brand, I have my own that I believe in, but I won't name it hear. If I did you would have about fifty posts to your message in about thirty minutes. Oil for some reason gets everyone chiming in with their opinion.
My comments could be considered opinion, but I hope you see that they are backed up with the results of high miles in your area of the country. Boy, do you really think we're looking at another drought this year?
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in