Car mileage w/synthetic vs conventional oil

I switched my 2004 Mazda 3 over to synthetic oil this fall and my mileage improved by 4-5 mpg for each subsequent tank of gas.

Naturally, I was quite happy. However, when I had my next oil change at the dealership, (different shop than the above), I suspect they may have used conventional oil. My mileage plummeted 4-5 mpg with each tank and as the weather got colder, the car is now running almost 10 mpg fewer. Also, the car runs a bit rougher. I have considered the type of driving and that has not changed since the summer. Mostly short trips, with about one long one, (over 200 miles) per month.

I don’t know how to tell what type of oil is in the car so I might approach the dealership with some evidence that they used the wrong oil. Any thoughts?


The sure way of finding out is to go back and ask them what kind of oil they put in your car. Otherwise, I really don’t know it there is a way of telling the difference.

In general a change in oil, synthetic to conventional or vice versa, does not cause rough running problems. In practice, I don’t get any of the mileage improvements in any of my cars when I change to synthetic and back, so I would look elsewhere for why the engine now runs rougher.

How many miles on the Mazda, and have you done your 30/60/90K services, per the owner’s manual?

Mileage in the winter will also go down, compared to your summer figures.

What about using a synthetic for a 75 caprice? will it help for less maintenance?

Just go ahead a change your oil again,if you think giving you the results you claim, you won’t be happy until you are sure syn. is in your crankcase.

How are you calculating your mpg? just miles driven divided by amount consumed? nothing wrong with that,accuracy is probably plus/minus 5%.

I have a hard time believing that just switching to synthetic oil will net a 4-5 MPG improvemnt. There is something else going on here to cause that much of a drop in MPG.

A .5 to 1 mpg improvement would be more like it.

I have learned to just let peoples mpg losses/gains go in one ear and out the other,also all the benifits/damages by this or that miracle product go the same place,pointing out facts about additives,octane ratings,ethanol effects just does not do any good. I feel like doing what one of the “regulars” on the Forum does,have answers ready for “cut and pasting”.

for the older cars in our family, we have been sticking with conventional, and a 5K oil change interval. First, They don’t all get driven enough to justify the extra expense for synthetic. Secondly, some of them eat oil anyway, so it is cheaper to pour in conventional and change more frequently.

Your choice, but I would just stick with conventional. The one vehicle I will always put synthetic in is the one that is the hardest and messiest to do the oil change, to decrease the number of times I change oil every year in it. The others get whatever is on sale and the best deal, synthetic vs conventional.

When was the last oil change? I would guess along with an oil change, the weather has turned colder and you may have winter formula gas now, both of which will reduce mileage.

Oil comes in two flavors. Original and synthetic. Synthetic will give a little better mileage (all else being equal) but I certainly would not expect that much of a change. Oil has a variety of colors (weight) and that would likely have a greater effect.

In both cases you should look in the owner’s manual and make sure that you always get an oil that meets those specifications.

All that said, I am curious about my mileage improved by 4-5 mpg for each subsequent tank of gas. There is no reason I can think of that would explain that in relation to oil. What was the actual mileage results for each tank? What was the weather like? Do you know what oil and what weight oil was used each time. Frankly I doubt if it has anything to do with the oil.

First off I seriously doubt using synthetic or conventional oil is going to have that big a difference in gas mileage. I’ve used both synthetic and conventional…and at best I’ve seen a .5 mpg difference…which I can’t exactly what the cause was.

There’s something else going on here…One thing to consider is that winter gas will get less mpg then the summer blend…considerable less mpg.

Thank you all for your responses. I will consider as I bring the car in for its next oil change!

It increased 4-5 mpg for each subsequent tank of gas until your next oil change?

Let’s see, if you have a 14 gallon tank and change your oil every 5,000 miles that would be 357 tanks of gas. Multiply that by 4.5 mpg and your gas mileage has gone up 1,606.5 mpg! Cool!

I suspect you meant 4-5 mpg (plain) or perhaps 4-5 miles per tank average?

I also suspect that the change was caused by factors other than the oil, like perhaps the improving weather or a seasonal change in fuel formulation at the station.

Please tell us the following:

  1. At what mileage did you have your oil changed; what viscosity e.g. 5w20 was put in? What was the PREVIOUS viscosity?

  2. How long (miles) do you go between oil changes?

  3. What SEASON was it when you got the good mileage? What season was it when you got the BAD mileage?

  4. Have you changed from tanking normal gasoline to gas with alcohol? If so, was it E90, E85 ?

  5. How do you measure your gas mileage? How many tanks do you average to get the figure? Do you tank at the same pump each time?

Unless we have answers to these questions, NO ONE on this panel can even begin to answer your questions.

In general, if you switch from synthetic to a normal oil of OF THE SAME viscosity you may lose 0.5 miles per gallon as mentioned.