When i bought my truck my gm guy recommended Castrol GTX 10W-30. After I got beyond 200k he said "put 15W-40 in it until it quits." I live in Tennessee where we occasionally get some cold winter and it works fine, so you should definitely be fine in texas.
BTW-Valvoline is really good stuff, I just personally prefer castrol.
NEVER use havoline or any other wax based motor oil, it will clog up your engine block with gunk. Also, If conventional oil has been used in the past, continue to do so (especially since half conventional-synthetic can be a little more pricey). The person who said the same earlier was absolutely correct, full synthetic will run right through your engine.
Also, I have been adding a quart of lucas for 50,000+ miles now and I think its good stuff. My truck might use 1/2 a quart of oil at the very most in 3 months...if even that. GM doesn't recommend additives, so its something you should definitely put some research into when you get into some high mileage.
2000 Silverado Z71 1500 Pickup 5.3L 4L60E w/ 4.10 locker, 285/75R16 BFG AT tires, Single In/Dual Out Flowmaster 70 Series Exaust. Rockford Fosgate 3x5 and 6 piece component system.
As of 8.7.08 215,000 miles and still running strong.
I ran that stuff (Max Life) in my 91 and got 150 reliable miles out of her before I traded her in. It even helped keep the known TBI engine valve issue at bay for the time I had her. (the last two cylinders had problems with oil leaking into them from the head, mine never got bad enough to kill the plugs or anything).
99' Silverado 5.3 4X4
Modified Predator 93 tune: TQM @ 40%, injector slope bumped up 4 levels,
125 MPH top speed, 5500 RPM limit,
TBS, 180 thermostat, 4" CAI with 9x6" element,
Bypass and De screen performed,
AVG: 16 MPG City, 20 MPG Highway
WEll I read every one'sa post and in my 1990 Chevy with a 5.7 I ran all sorts of oils in her courtisy of Shell coorporation and my father being able to get his hands on free oil. Anyhow I do not recall who said that if you use conventional oil and then run full synthetic it will run right through your engine... I disagree with that. As I said before I have ran conventional, half and half and full synthetic and even some oil from Europe that is not for legal sale in the USA and I never had a problem. In fact the only time I usemore than half a quart of oil in it was when I ran Quaker State and a bottle of Slick 50 when that fad was happening. I used a full quart. Other than that I have ran all the above and even the Walmart Tech 2000 with know problems. As long as you change your oil and filter at the 3,000 or as I do the 5,000 mile mark there is know problem. And I sold my truck withroughly 207,000 miles on it. And the guy who bought it from me is still running the same motor with conventional oil at 3,000 mile oil and filter changeswith over 300,000 miles on the original motor. It still uses only half a quart between changes. Oh yeah and I have ran both 5w30 and 10w30 and 10w40 and I could not tell a difference. My truck now I run 5w30 and after a 100,000 miles I will probably switch too 10w30 depending on how much oil it uses. Now my dad is a believer in the High mileage oil in his 90 model chevy with over 350,000 miles on the original motor... tired, but still running.
2004 GMC Sierra Extended Cab- Sold
2014 GMC Sierra- Stock