in texas, its best to use a bit thicker oil, because as stated, the warmer the engine the thinner it gets, and down in the sun belt we get plenty of heat. 10w-30 or 10w40 are great in texas through the summer months (and in houston, extend that through april-october).while in the winter, its best to drop the viscosity down a touch since it doesn't get as hot and you are experience more cold weather. in that case, its best to go with a 5w oil, 5w-30 is best, I wouldn't go much lower than that. personally, I use the 10w-40 in the summer, and 5w-30 in the winter on all my cars.

for people up north, an even thinner viscosity is not unreasonable, like a 0w oil in the winter.

as far as synthetic compared to dino, I like the dino stuff, but I have older cars, like 1987 and 1994. as stated, synthetics do much better in new cars, that don't have leaks. I just hope you're not the kind of person (like my friend's step-dad) or thinks that just because you have synthetic in there, it can go 10,000 miles w/o an oil change. even with synthetic, its best to change the oil no more than 6000 miles tops. dino oil should be changed every 4500-5000 miles tops on cleaner engines, both types of oils, if the engine has any kind of age/sludge, should be changed every 3500-4000 miles.

if your engine DOES have sludge, a great engine cleaner is Seafoam, it really gets the gunk out of the engine. Just follow the directions on the container.