Effects of using wrong oil weight?


My 2007 Honda Fit requires 5W-20 oil, but I recently changed the oil and put in 5W-30 by mistake, since that’s what my other car takes, there was a special at the auto store, etc.

The engine seems to run fine, but I have noticed a slight decrease in mileage – I usually get ~30 mpg (combined hwy/city), and in the one tank of gas I got since the oil change, I noticed it was about 25 mpg.

Should I drain the 5W-30 and replace it the correct weight? Am I just looking at reduced gas mileage, or could I be causing damage to the car (through oil deposits or whatever) because it’s not engineered for the heavier weight? And what’s with oil weights, anyway? I thought the lower number was the most important one, in terms of startup viscosity, etc.


Todd Bernhardt

The base weight of both oils is the same. It’s the ability to maintain viscosity (resistance to flow) at higher temperatures that’s different. The 5W30 has more “viscosity modifiers” that the 5W20. Viscosity modifiers are actually coiled polymers that uncoil as the oil heats up and affect its ability to flow. The problem is that the modifiers don’t lubricate as well as the base oil does.

Look in your owner’s manual. They may both be optional choices. If not, I’d drain and refill, only because I’m picky and I know Honda is too. You can sleep soundly, however, you’ve done no damage. The base weight is correct and the difference between the two is minimal.

There’s a good primer on oils at www.carbibles.com. I recommend a visit.

I wouldn’t worry about it. It might make the difference between the car lasting 300,000 miles or only 299,999 miles.

Don’t worry about your 5W30 oil. I doubt if the change from 5W20 was responsible for the drop in gas mileage. The only accurate way to compare mileage is:

  1. Use the same gas at the same station AND THE SAME PUMP! Stop at the first click each time! Don’t try to top up the tank. Beware that each pump is different.
  2. Only compare the same kind of driving patterns; city, highway or mixed.

For instance with my Toyota I have had gas mileage range from 45mpg to 32mpg reflecting different driving styles.

Hondas in foreign countries use different oil in the same engine. In the tropics my neighbour’s Honda required 20W50!!!Only the US has mandatory EPA CAFE mileage standards with penalties. Therefore US built cars are pushed into the thinnest possible oil resulting in the highest mileage, but at substantially shorter engine life.

A few weeks ago we had a simlar posting, and various respondents commented that a) modern engines have tighter clearances, b)modern oils are better quality. However, the best way to ruin your engine is to fill it with cheap 5W20 oil, and set out for Death valley in July. If you really want to use 5W20, go synthetic, Mobil 1 or similar quality. North American manufacturers balance warranty costs due to very cold starts and quickly loading the engine with possible fuel penalties if they sell too may big cars.
So the 5W20 decision has little to do with well being of your car and everything with the manufacturer’s bottom line profitabilty. If I was in your shoes I would use Mobil 1 synthetic 5W20 until the engine warranty was up, and then switch to 0W30 (yes, there is such a thing)synthetic in the winter and 5W30
synthetic in the summer.

Thanks, TSM. Just to follow up to your points, I did check the manual right away, and they said only the 5W-20 was specified. Appreciate the link, too. I will check that out this weekend.

The oil weight is not responsible for a 5 MPG loss in mileage.
Cooler weather and gas reforumlation may be the cause of that.

You can forget about it until the next oil change. Then go back to 5W-20. No harm will come to the engine. NONE. One tank of gas is not enough to say you’ve suffered a mileage penalty, and the difference between oils is not enough to account for 5 mpg anyway. Something else reduced the mileage, not the oil.

Drive your Fit and enjoy it, just buy the specified oil next time.

Thanks, Doc! I knew I’d have to wait a bit for several fillups to see if my mileage had actually changed – my driving’s pretty regular, though, so that’s why I wondered. Other than the oil change, there was nothing different I did before that fillup.

Having once been a proud and happy owner of a diesel Rabbit, I was used to changing oil weight according to the seasons. Didn’t realize some of the other stuff you mention – thanks for that (0W30 … whoa!). The funny thing about this whole mixup is that I was buying synthetic – they had a good deal on it, and I just spaced out and grabbed one weight, forgetting that the two cars took different weights…

Thanks to all of you for your responses!