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My 2000 5.9L R/T says "Premium Only" inside the gas door. I asked the guy I bought it from about it and he says he never put anything but the cheap stuff in. I've done the same and never had an issue since I bought it last fall. I've never even heard a tick. Now, have I given up some power from this? Perhaps. But I'm OK w/ that. So, technically, yes you are supposed to use it. But if you don't nothing bad will happen... At least not from what I've seen.