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Here's the facts. I work for an unnamed auto service chain and we put bulk 10w30 into Civics and Accords all the time. We see these same vehicles in time and they show no signs of problems developing from this. In fact, the oil is often cleaner than the serviced 5w20 or 5w30, for the obvious reason that those are lighter and therefore less protective. As mentioned early, manufacturers recommend 5w30 or some other to improve the gas mileage that they can post on the window. There really isn't that great of a difference among these engines that small changes like this are causing them to go wild. With one small Toyota I have, the four-cylinder is so hard on the 5w30 that you can tell its not the best oil for parts wear. But that isn't necessarily the manufacturers first concern. When you work with oil every day, you realize that folks really worry far too much about lubricant. Being careful is always advisable, but fretting over 5w20 vs. 5w30 is somewhat like worrying whether your shirt is cotton or cotton/10% polyester.