My wife just purchased a 2003 Mustang with a 3.8L, V6, with 24,000 miles on it. It calls for 5W-20, but the previous owner took it to Wal Mart and they Installed 10W-30. I plan on using the specified oil 5W-20, but an unfamiliar with it. I checked around and can find some 5W-20 (Castrol) and several containers of 5W-30 (Valvoline, Havoline…) Should I stick with the 5W-20 or would the 5W30 be OK? Any suggestions on brand?