Plugs for sure, 72K change for mine... made a world of difference. i'm at 87k now. for a 6.2L with 300K... wow, getting close to end of life territory. maybe add the magical high mileage sea foam in it! add it to the oil and fuel. I used it (fuel and engine oil) in our 6.2L shop truck with 200k and it purs like a kitten. no expert on it and i'm sure there are people out there that disagree but at 300K and loosing power, I don't think it is a bad option to consider.