I have a 2004 Mazda 6s and my oil fill cap states to use 5w-20. I have used 5w-20 every since I have owned the car, but recently switched to 5w-30 since I just passed 100,000 miles. I will probably switch to 10w-30 once I hit 130-140,000 miles. I just want to slowly progress to thicker oil as the car gets older, seals start to go bad, etc. To answer your question, there is really such a small difference between 5w-20 and 5w-30. The 5w-20 was specified by Mazda, so that they could squeeze out another .333 MPG due to federal regulations (less friction than 5w-30 oil). I would just go with a 5w-30 oil as it will offer slightly better engine protection, and you aren't going to notice the .333 mpg decrease. As for what oil, I would recommend Mobil 1, but you will probably hear 10 different oils from everyone else on this forum, and they will all probably be good oils too. I would STRONGLY recommend going with a WIX or MOBIL oil filter if you have them available in your area. I was using FRAM high mileage oil filters for awhile, but recently realized how crappy they are. I switched to WIX, and the moment I held the filter I could feel how much better made it was. My oil does not seem to get dirty as quick with the new WIX filter. And yes, you need 6 quarts.