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replace/maintain the car with oem or equal parts. I don't really belive that using $15 spark plug is worth the gain some compaies claim. It's more important to have the car run right, not so much running with updated, expensive parts.
I have never heard of people changing oil a little bit at a time. If .5 liter of oil inside an engine is dirty it will just spread to the rest of the oil so i just don't see how that works. I change my oil every 3000 miles or so.
I would look at making your car more aerodynamic and keep it at 60mph or less if you can on the highway. I think the best mpg is at about 45mph
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