why when you do an oil change you get less miles per gallon?

Question

i am using mobil extended performance that goes 15,000 between oil changes. When i put it in new i get 14 in the city and 20 in the highway but over time, right before the next oil change i get 18 in the city and 24 on the highway...

Answer 1

i never herd of that happing before sorry

Answer 2

Thinner oil gets a little better mpg. For instance my Honda recommends 5W-30 up to near 100 degrees. Something not normally recommended in temps so high. They are counting on thin oil to help you get the most fuel economy.
Conversely if you use certain oils that are thicker when new then you may see a small difference in mpg. I have never seen more than 2 mpg difference myself. I always use multi grade oils to avoid this as much as possible.

Answer 3

Funny, I always get better mileage, unless I changed to a thicker oil, or changed to a cheaper brand.
I have noticed a 2 mpg difference between brands, the Havoline I am now running boosted the mpg 2 miles in town and 3 on the Highway.
So my guess is that you changed your grade of oil or brand and it was not only the oil change that resulted in your observation, but what you used.

Answer 4

my idiot ex wife used to think the car ran better after changing the oil. its all in your head. your not driving the same when you check your mileage. the difference would be negligible and certianly not 4 mpg. the air in the tires and how much weight in the car has more of an effect on your mileage than an oil change

Answer 5

Because the oil is thicker when it is new. When the oil gets thinned out it moves easier but protects the bearings less.

Answer 6

I can't say that I've noticed a difference in my truck. Are you using a synthetic oil?

Answer 7

The two have nothing to do with each other at all.

gas burns -

Oil lubricated.