what kinda of oil do you use?

i went to a texaco express lube to get my oil change and i told them to put 5w-30 in my car thats what it said under my hood on the sticker. i have a 1999 atx and i live in jacksonville fl, he told me that he would not recomend that oil for my car something like it turns to water when it gets to hot. so is this true....