Changed Oil and Getting Worse MPG!!

I did my research and looked at old threads and decided to go to a Synthetic Oil and a K&N Oil Filter. My H3 had 5555 miles on the factory oil and filter and my MPG was around 15 MPG. I changed my oil myself and I am now getting 11 or 12 MPG. I really don't understand why the big drop. The only diffrence is that my H3 was one of the Hummer that ran a little warm( I rember a thread talking about the diffrent thermostats the H3 plant put in for Northern and Southern delivered H3's) now that I have changed oil the Temp guage stay's put. Even hauling a trailor does not move. I will proberly go back to the stock AC Delco Filter and regular 5w 30 oil.

Which brand of synthetic did you use?

what weight oil did you use? I have never heard of such a big drop in mpg from an oil change. that means I'd be down to 5 mpg if I changed my oil

The H3 is supposed to run Synthetic 5w-30 oil. The dealer told me this, and the two different oil change places I went to told me they wouldn't put standard in because GM says it's synthetic-only. You did the right thing. Maybe those K&N filters are limiting the oil flow and causing alot of friction, but then it would be hotter. I'm confusing myself now!

Assuming that you have the correct grade oil, the K&N filter could be more restrictive than the stock oil filter or you could just have a bad one. If so, it would put an increased load on your oil pump that could cause reduced mileage. This wouldn't really increase your oil temp but at high revs it could cause the filter's bypass valve to open which would negate the filter's ability to filter your oil. Not a good thing.

I'd stick with the factory filter for all future oil changes because it's the only one that has actually been tested and approved in a Hummer by Hummer.

In my days as a staff engineer with a competitive brand I've spent many months away Summer testing out in Beaty NV, Pheoniz, and Houston. GM also tested out there too and their crews hung out in the same places after work hours as I did. Anyway, they always seemed like a really sharp bunch and I'll always trust what they recommed.

Very good advise..I also use only a GM filter PF-61..

I went with Castrol Syntec Oil 5W 30, The only reason I went with the K&N is the Price the other filters were running around $9.00 and the K$N was $11.00. I will most likley stay with the Synthetic Oil and try changing the Filter back to the AC Delco. Other than the GAS mileage it seems to be running great.

I've used Castrol oil for over 30 years and since 2002, I've used nothing but Castrol Syntec...I have never, ever, ever, had a single engine problem..no seal leaks, no smoking exhaust, no bad valves, no lifter noise..nothing!!!!..So in my proffessional and personal opinion..you are using the best oil known to the civilized world.

i've noticed mileage quirks from oil changes, old oil (getting towards time to change), and on certain tanks of gas. Its tough to say for sure that mileage is up or down 1 or 2 mpg, as wind and temp and all can vary so much, but i've never seen anything like 4mpg difference.

I recommend a steady diet of Mobile 1, the factory air system (K&N and similar filters can cause problems with the electronics in the motor, and modern engines have good intake systems that minimize the value of such aftermarket products), and gas from a source you trust.

K&N are great filters and are oiled properly from the factory...The trouble arises is when people improperly clean them and don't let tem dry completely and then over oil them and don't let them set long enough before re-installing..It's not the filter that's causes problems with the MAF, it's improper execution of the maintenance routine in cleaning them.

I got it, Dennis. I had some over-oiling problems with a filter once upon a time, so don't take my word on the filter thing as i'm not competent enough to oil them properly.

That's cool Blade...I didn't mean anything derogatory or insulting towards you...It's just that people get the wrong impression of this good filter when someone has over oiled them and blame the filter as the culprit for the fouling of the MAF...

Is the GM factory filter for the H3 made by AC-Delco? Can you buy it at a Pep Boys or similar place, or do you have to get it directly from the dealer?

I'm sure it's AC Delco. BTW, my local Chevy dealer (about 1/2 a mile from home) sells them for about $4 so I stopped making the 10 mile run to Autozone. My local ShopRite grocery store also sells Mobile1 for about as cheap as I've ever seen it anywhere so that's where I buy my oil.

Considering the new "Palace" that Hummer built in my area, I wonder what they are charging?

I have been following this post and it has me stumped. In every new vehicle I have ever owned, I seemed to get a little better fuel mileage with my first oil change. Not saying it was the oil change itself, as I always thought that it was mostly the vehicle being broke-in as to why it got better.

I had a K&N Cold Air intake installed in my 2000 Nissan Frontier and had problems with the MAF from day one. The Check Engine light would come on if I had the airconditioning running while the vehicle was running at idle when sitting in lines like at the bank, long lights, or other types of long idling. I had to turn off my air to stop the light from coming on. Was a pain. With that and the sucking noise, it really was an attention getter. I would draw strange looks where ever I went. I did gain between 20 to 45 miles per tankful of fuel and that was a plus. When I traded my Nissan in on my H3, my 16 year old daughter made me promise not to put one on the H3! The sound to her was an embarrasment. I did order a K&N air filter replacement for my H3 as I had one on my wife's Mercury Sable and it worked good without any trouble on the engine light coming on.

I am planning on using K&N oil filters with Castrol Syntec oil and maybe after 9000 miles, may try Royal Purple as my son was telling me about it about a year ago and said he heard it works good. My brother-in-law also uses it in his BMW. I have 1600 miles on my H3 and plan on changing the oil at about 2500 so I will let you guys know how my milealge changes for better or worse.

I have been using syn oil with K&N oil filters for a long time, many years before I got my H3. I just can't see either the oil or the filter causing a loss in MPG.

Have you noticed any other things like, any smoke on start up, hard idle, louder whine on acceleration? Could be one of many things that could casue this drop in MPG. Make sure tire pressure is up, none of the brakes are dragging, also don't switch gasoline brands for a few more tanks while you keep track of your mileage.

BTW, I use Amsoil and K&N oil filter.

Good luck let us know what happens.