Weird. I change my oil pretty often (every 4000 miles, which lasts me < 3 months). After each time, my MPG goes from approx 26 down to 21. It gradually increases as the weeks go by, but when it comes time to change the oil again - same problem.

Nothing different in my oil changes. New OEM filter everytime and Mobil 5w30. Anyone else experience this?

 

Is it possible that you drive a bit harder, knowing that you have fresh oil? I can think of any other reasonable explanation.

 

no, unless u had some weird oil in there that was not energy conservant, i duno

 

That sometimes happens to me too. I get the best gas mileage towards the end of my 5K interval.

 

Wild guess, sheared down and/or fuel diluted oil into the 20w range.

 

A family friend is my mechanic, so he wouldn't do anything as petty as diluting the oil.

I've tried this out for the past 15 months, which consisted of about 4 or 5 oil chianges. Almost everything is consistent:
1. gas (super 93) from BP, across the street from work
2. same Mobil 1 oil, new OEM filter during every change
3. records taken only during normal driving (spirited driving occasionally, but I don't measure MPG for that tank)
4. car is only driven back and forth to work 60 miles/day (weekends is a different car)

Just changed my oil last Wednesday, filled up last Thursday. Got 21 MPG for that tank.

 

Dude, don't hurt yourself. Don't show this thread to your trusted family friend mechanic. He may take you for a ride next time you visit him.

Hint: fuel dilution happens in the cylinders during combustion, not during an oil change...

 

Asumption:
As soon as you change the oil you are topped off on oil. As the car is being driven for 3000-5000 miles oil is burned & consumed (many cars can use up to 1-2 quarts between oil changes) less oil for the crank to splash though less resistance on the motor better fuel the farther you get from new oil changes.

 

Because I had an 92 galant that burnt oil like mad and even that would never amount to 1-2 quarts. If the oil gets down that low in a V6, you're in trouble.

Also, lubrication gets worse with oil age, so MPG would go down in 3000-5000 miles.

 

Really,

My brother bought a sentra se-r from a friend of ours after is was wrecked. I was over there one day when my buddy was changing the oil a he took the drain plug out and about a quart of oil came out total between the oil filter and drain plug. The oil light never came on him while driving before the oil change. After my brother bought the car we put the an SR20DET in the car and torn down the old motor that ran on less than 1 quart of oil (many times). To our surprise there was NO damage any where the motor ran great when we took it out it had around 179,000 miles on it.

Most cars will not be hurt running 1 quart low on oil.

 

I4 and V6 have approximately similar oil capacity. According to you I can cut my oil costs by around 75% by only using 1 quart of oil in my engine. You cant be serious.

 

Ya sure let try it on your car first. No I don't advise running 1 quart of oil.

Yes some I4 hold as much oil as v6. My RSX-S holds a little over 5 quarts and every v6 I have ever worked on held between 4.5-6 quarts.
(Yes a 2001 Impala 3.8 V6 holds 4.5 quarts of oil)