03PoneyGT

I bought my car a few months ago and I've always changed my oil on my own putting 10w30 in my old car, now with my 03 stang gt I get free oil changes for live so I was like sweet. BUT, as I checked the oil I noticed I was a qt short! I went to the place that does the oil change and I asked if they put 5w20 in the car being that's what it says on the cap, and they do..

Now from my experience I notice that 5w20 is as thin as water. Should I put 10w30? or 40 being that it's getting warmer? What do all of you put in your stangs?