I have heard that a car that takes 5w30 and runs 10w40 will perform/consume differently, enough to make a difference. I have no idea. The general rule that most people I have spoken with follow is to use the grade the manufacturer has set for your car. What year and model car do you have? Are you running synthetic?

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1989 300CE Astralsilber/Schwarz
1992 300CE Schwarz/Schwarz