My 98 Bonnie dropped from about 25 mpg to about 23 mpg since I put in 10w30 instead of 5w30. It has 170k miles. Tire pressure is 35 psi recent chg oil,filter,air filter,gas filter, whats up ?

How many tanks of gas are we talking here? Could be bad gas, natural drop due to dirty engine internals, etc....

I can't see how changing from 10W to 5W would matter....

isnt 10 w 30 recommended anyways?

the last 5 or 6 tanks full. A local shop guy said the oil would do it. He said thats why alot of the new Ford trucks recomend 5w20,Ford hadto go to this to get the most mileage.. I always ran 5w30 before but the motor seems quiter with 5w30.. What do you guys see for mileage ?? I do alot of highway too,seems I should be more like 26 mpg

I was using 5w-30 during the winter and then 10w-30 during the other month. When I switch I didn't notice any difference. But a series 2 motor does work differently then a series 1 not sure if those difference would have roll

I seriously doubt that your decreased milage was do to the oil change. And although some manufactures(like Ford and Honda) have gone to a 5W-20 oil, it'* mostly becuase of the tighter tolerences in their engines and, yes, they do get mayyyybe 1 mpg increase in mielage over heavier weight oils I know that VW recommends 5W-40 Synthetic Oil in their vehicles. It might be time to change the 02 Sensor and plugs. But give it a few more tankfulls to see if you milage improves.

You should see no performance change from 5w-30w versus 10w-30w.

I would instead start looking at how your car is running and do you get any rotten egg smell when sitting in traffic for a while. The rotten egg smell would lead you to change the O2 sensor before it wipes out the catalytic converter.

The O2 is typically the biggest factor in lost gas mileage.

I agree, i HIGHLY doubt your oil has anything to do with it. I run 10W40 in my car and i get crappy mileage, but that'* the car not the oil . Besides 2 MPG isn't a big deal; slight variances in your driving, road conditions, temperature, etc can have an effect on your mileage. But if it really bothers you you can pop a new O2 sensor in there like was mentioned earlier. For ~ 40 bucks it'* a good investment anyways.

it shouldn't make a difference in the summer.
Winter I use the 5w-30 cause it gets down right cold in new england, sometimes 0-5 above. Thats when oil will loose you mileage.

I read online that the oil companies would be refining "winter blend" gasoline now, due to the Hurricane, as it is quicker and cheaper to produce. I also know for a fact, that I always get less mileage in all of my vehicles due to this "blended" oxygenated gasoline...I wonder if that'* happening already?