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Discussion Starter #1
WTF! I changed my oil yesterday. I normally run Castrol GTX 5w-20, 2009 Hemi, 19,000 miles

I decided to try synthetic so I went with Mobil 1 5w-20 and a WIX filter.

I drove a 300 mile round trip yesterday evening that I have done many times before and I always average around 17.5mpg for the entire 300 miles. This trip was different, i barely got 16mpg

I ran the cruise set on speed limit for the whole trip like I always do, there was no wind, very little traffic. 70mph on the highway, 55mph on the back roads, only one town to drive through. Conditions were about the same as always. Fifty degree sunny mountain weather!

I would have thought that the syn would have at least netted me the same mileage if not better.

I was told by my dealer that the MDS/VVT likes the syn better and will hold in 4cyl mode slightly longer with it.

I'm going to try the syn for one more change and if I continue to see the mpg drop I'm going back to dino oil.

Anybody else have this issue?
 

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Where was the moon? The gravitational pull of the moon will effect your gas mileage with a vehicle as heavy as the xk!
JK, but it could be tire pressure, your air filter, any number of things... I wouldn't worry about it much, check the obvious things and if it doesn't get better then dig deeper.
I didn't notice a drop when I switched. I'm sure someone here may have some good ideas.
Keep us posted!
 

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Before lifting and bigger tires I could get that if most of my trip was on a 55MPH two lane. Maybe I still could if I really tried...
 

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Discussion Starter #5
Where was the moon? The gravitational pull of the moon will effect your gas mileage with a vehicle as heavy as the xk!
JK, but it could be tire pressure, your air filter, any number of things... I wouldn't worry about it much, check the obvious things and if it doesn't get better then dig deeper.
I didn't notice a drop when I switched. I'm sure someone here may have some good ideas.
Keep us posted!
I checked tire pressure before I left, 36 in all 4. The air filter is new as of two weeks ago, its a WIX.

I did rotate the tires while I had it in my garage but that is it.

I normally get 19 on constant highway runs but this trip(one that I do often) is a mix of interstate in the mountains(I-81S to I-77N) which crosses the eastern continental divide then west on 52(byway) into the coal fields of VA.

I'm still puzzled by the result
 

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Discussion Starter #6
Before lifting and bigger tires I could get that if most of my trip was on a 55MPH two lane. Maybe I still could if I really tried...

I was getting ready to lift mine but I'm talking myself out of it for a couple of reasons. One is gas mileage and two is for resale value if I ever decide to sell it.
 

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New synthetic oil may be cleaning out the stuff from the old type oil. Oil number 2 should bring it back up. Also change it at regular interval for the first 2 or 3 oil changes before going longer


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Discussion Starter #8
New synthetic oil may be cleaning out the stuff from the old type oil. Oil number 2 should bring it back up. Also change it at regular interval for the first 2 or 3 oil changes before going longer


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Yep i plan on that but still doesnt explain the mileage drop. I only have 18k on it and I've always changed out at 3k so it shouldn't be too dirty.
 

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The only thing I can think of is you rotated your tires. I guess a simplistic view of this would be more friction in tread. In theory moving the tire means you might be wearing down high spots in the tread which means more friction going down the road. That's what I came up with LOL I know it's thin but its a theory.
 

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complete speculation here...


I've heard our HEMIs are very picky when it comes to oil weights and allowing the vehicle to switch in/out of 4cyl ECO mode. Maybe one brand of oil is slightly different, and the outside temp was different and those two factors combined and kept the motor from switching to 4cyl mode as often.
 

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More speculation - is it possible that the Mobil 1 never hit your crankcase, i.e., did you actually add it yoruself?

Is it possible that you've just switched over to winter blend fuel where the stoichiometric ratio is a little lower than the summer blend? Winter blend oxygenate additives such as MTBE displace 'gasoline' in a given gallon of fuel, and because they contain oxygen, require more fuel consumption for even burning and emission control.
 

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Discussion Starter #13
More speculation - is it possible that the Mobil 1 never hit your crankcase, i.e., did you actually add it yoruself?

Is it possible that you've just switched over to winter blend fuel where the stoichiometric ratio is a little lower than the summer blend? Winter blend oxygenate additives such as MTBE displace 'gasoline' in a given gallon of fuel, and because they contain oxygen, require more fuel consumption for even burning and emission control.
Yes, I change my own oil.

Winter fuel is a possibility, I filled up before the trip and topped off at the same station I always stop at. We have had really warm weather for late fall so I really didn't think about the winter mix yet.
 

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Changing oil type or even brand can affect how an engine runs. Way back in the 1960's I switched to Royal Triton Purple oil - about 100 miles later all my rings broke. The reason is my engine parts were used to moving and flexing the same way with the oil I had been using for many years. Adding the High Detergent Triton oil loosened up the rings and they flexed different and broke. (Chev small block) A super master mechanic explained that - he was also the one that sold the Triton oil:)
Since then I always use the same Brand and Weight in my engines. (9 cars since then using Quaker State with no failures due to oil) I run 5W-20 Quaker State in my Hemi, Average 17.5 MPG highway. Have recorded 21MPG with a tail wind driving 70+MPH for 100 miles. I installed the MDS LED and it comes on a lot showing running on 4. See post #45 up on this thread http://www.jeepcommander.com/forums/showthread.php?t=10433 on how to add.
I'm not pushing Quaker State, just saying stick to the same oil all the time.

Can synthetic change things ? My vote is Yes.

I have been professionaly repairing mechanical and electronic equipment for over 45 years - 30 of those as a level 3 technical services specialist.
 

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Discussion Starter #15
Changing oil type or even brand can affect how an engine runs. Way back in the 1960's I switched to Royal Triton Purple oil - about 100 miles later all my rings broke. The reason is my engine parts were used to moving and flexing the same way with the oil I had been using for many years. Adding the High Detergent Triton oil loosened up the rings and they flexed different and broke. (Chev small block) A super master mechanic explained that - he was also the one that sold the Triton oil:)
Since then I always use the same Brand and Weight in my engines. (9 cars since then using Quaker State with no failures due to oil) I run 5W-20 Quaker State in my Hemi, Average 17.5 MPG highway. Have recorded 21MPG with a tail wind driving 70+MPH for 100 miles. I installed the MDS LED and it comes on a lot showing running on 4. See post #45 up on this thread http://www.jeepcommander.com/forums/showthread.php?t=10433 on how to add.
I'm not pushing Quaker State, just saying stick to the same oil all the time.

Can synthetic change things ? My vote is Yes.

I have been professionaly repairing mechanical and electronic equipment for over 45 years - 30 of those as a level 3 technical services specialist.
Being a mechanic, you will not convince me to run quaker state. Ive seen way too many issues with that stuff. I draw the line there. I say good luck to you.

I will run Castrol in the newer Jeeps and Shell Rotella in my 4.0 XJ's/MJ's.

Back to the issue at hand. Our modern engines are designed to run on synthetic more so than dino oil. I don't see what the harm is in changing at 18k after soaking the engine with plenty of zinc/phosphorus for the last two years.

The synthetic should allow the engine to run cooler with less friction, that all adds up to more efficient.
 

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Did you use Wix Oil filters before with the other oil? A better filtering filter may create some pumping losses, but yea, a measureable amount up in 1.5mpg, I just don't see it.
Yes, I change my own oil.

Winter fuel is a possibility, I filled up before the trip and topped off at the same station I always stop at. We have had really warm weather for late fall so I really didn't think about the winter mix yet.
The EPA makes them switch over to a Winter Mix of fuel, I don't know for sure, but I doubt its based off the temps the last few days, I'd think it would be certain dates a year, the depot is required to only sell a certain mix of fuel to the filling stations.

That different mix could account for a 1mpg or more.

Air temp could account for a change as well, but I'd think cooler would be better mileage, NOT worse, but I could see an argument where it might be the opposite.

Did you check your Air Filter, that can have a huge effect. It only takes driving through one dust bowl for a few minutes to clog up an air filter and drop your mileage.

I can't see why switching to a good synthetic like Mobil1 would make your mileage drop, I would chalk it up to a coincidence and keep monitoring and see if it comes back up to mileage you normally get. There are so many variable that will effect fuel mileage, I wouldn't automatically assume changing oil caused a drop, there is likely another explanation that is alluding you.
 

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Discussion Starter #17
UPDATE;

After almost one month and about a thousand miles my mileage has improved. I'm getting almost a mile more to the gallon in town.

The Hemi seams smoother/quieter than on dino oil.

I may go Royal Purple on the next change just to see if there is any more improvement.
 

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Changing oil type or even brand can affect how an engine runs. Way back in the 1960's I switched to Royal Triton Purple oil - about 100 miles later all my rings broke. The reason is my engine parts were used to moving and flexing the same way with the oil I had been using for many years. Adding the High Detergent Triton oil loosened up the rings and they flexed different and broke. (Chev small block) A super master mechanic explained that - he was also the one that sold the Triton oil:)
Just to point out, engine built in the 60's used cheap rings, they broke all the time and wore out quickly. Todays engine use much higher quality parts, especially rings, that don't break as easy or wear out as fast. So, someone switching to Synthetic Oil today would NOT have to worry about their rings breaking in a modern engine, like a Jeep Commander.

That just to avoid people worrying about breaking rings, I'm NOT disagreeing that switching oils can cause changes in the engine, I can't disagree with that logic, although small you've got a point a change in oil could cause some sort of unexpected effect.

If a change in oil had any effect, I tend to think the adaptive computer controls on the engine are behind why some small change brought on by a different oil, caused a 1mpg decrease and then the adaptive controller adapted and brought it back to what it should be plus an extra MPG.
 

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I drove a 2008 Commander Overland from South Florida to Colorado and got an honest 17.6 for 2250 miles. I limited the speed to no more than 5 mph over the limit. It can be done.
 
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