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Has anyone with the ecotec noticed a big decrease in MPG when the car is due (or overdue) for an oil change? I have found that when my car is fresh off of an oil change, the highway mileage is up around 35-40, while when the car is about 3,000-3,500 miles into a batch of oil, the efficiency drops down to 30 in the same driving conditions. Just seems strange to me.
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