I use either Quaker State "Q" full-synthetic, or Mobil 1 Sythetic. Unless you're motor is already starting to smoke/knock/etc.. from other serious mechanical problems, I would run 5w30, especially in smaller engines. You can actually feel the difference between 10w30 and 5w30 in, say, a 2.2L Cavalier! I mght go along with the 0w30, if I built the motor myself, and knew it had pretty tight tolerances. Without knowing the previous history, I'm not comfortable going quite that thin, especially in the summer.
The Mobil 1 oil is $22/5 quarts at Walmart. Again, as long as your motor is mechanically sound/not misfiring/running rich, then you should be fine with 5-7,500 mile change intervals. Also be sure to use a decent filter, not the Walmart brand, or STP,etc... Personally, I like the Fram Tough Guard (Grey body, with the "grippy" stuff on the end). There will prob. be a bunch of guys jumping on here in 3...2...1..., to bash Fram. I've used them for approx 15 years, on many, many vehciles (including a 6-7,000rpm dirt stock car), and never had single oil-filter related problem.