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Was just wondering. I was noticing that I was getting lower mpg coming up on 10,000 miles on my 2012 golf tdi, and sure enough, right after the oil change I got about 15% better mileage. Is this common? Is it more common before the first oil change maybe? Is there any harm in changing the oil slightly sooner on future oil changes? Say 8000 miles apart maybe?

Thanks in advance.
 

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IMO, the only "harm" in changing your oil before the recommended service interval is to your wallet. As for the mpg, I didn't notice an mpg decrease prior to an oil change nor did I notice an mpg increase after an oil change. Currently at 27K so I've been through two oil changes.
 

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In my experience, the only time MPG would go down because of oil? When there wasn’t enough engine oil! Humans like driving with the engine oil barely on the dip stick because as long as the dip stick has a faint black oil mark… I mean who adds oil after an oil change?

This alone caused significant MPG loss. Why?
Engine oil MUST be present, and clean, to seal the rings to the cylinder wall. No seal? Fuel goes directly into the turbo, and rest of the exhaust system and out the tail pipe. This I burned fuel causes loss of power and money.

The engine must have plenty enough oil to remove diesel carbon build up (clean), reduce friction (lubricate), protect metal parts from excessive heat (cooling), remove oxides and inhibit corrosion.
Oil is the blood of the engine.
Diesel engine oil has far more of dissolved solids compared to gasoline engines. Those solids can clog the oil filter causing it to collapse and pump all that dirty oil into your shiny engine bearings… and more, unexpected, issues begin.

This has caused fuel economy issues along with clogging exhaust systems, turbos, sensors.

Change your engine oil according to the manufactures recommendations or at least once a year.
 
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