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.