What is the appropriate motor oil for 2003 galant?

10W-30 is the preferred oil, but if you live in a cold climate region, you may want to put in 5W-30. The 5W-30 will be easier on your engine when it's very cold (like below freezing) I would also recommend using synthetic oil vs. standard oil. It WILL definitely prolong the life of your engine, you won't be prone to engine sludge, you won't have to change your oil every 3000 miles (more like every 6000-7500 miles) and it's not near as hard on you engine during cold starts. The synthetic motor oils always leave a small coating around your engine components unlike standard motor oil that just drains into the oil pan when your engine is shut off. Synthetics are more expensive and I would go with Mobile 1 or Castrol pure synthetic. You can always buy your own motor oil & filter and take it into the garage of your choice to have the labor done. That will save you some money. Instead of 25-30$ for an oil change (if the shop supplies the oil and filter) most places will charge 60-75$ for the synthetic, that's why buying your own oil and filter will save you money and most places should be fine with that. (that's what I do!!!) You can also change from regular motor oil to synthetic at ANY time, even if you already have over 100K miles on your car. But you cannot switch back and forth once you go synthetic and you should use the same brand as well.