The only way to really mess up is to run the car without any oil.
One could, without any damage at all and in a pinch (let's say) run 2.25 qts of 5W-20 and 2.25 qts. of 5W-30. You'd have exactly 4.5 qts of 5W-25... seriously.
Unless the weather where you are is consistently below freezing for a couple of months, then 5W-30 is recommended for high-performance use with 5 or 10W-40 for summer high performance use.
5W-20 is now being used by most major automobile manufacturers so that they may be able to better meet their Corporate Average Fuel Economy requirements.
A thicker oil is just slightly harder for the oil pump and therefore creates more parasitic lossses in HP and this affects fuel economy slightly. Multiply this .3-.5 MPG times the number of vehicles that an automaker may sell and the savings add up.