So I did an oil change and decided to upgrade to synthetic oil. So I did and put a new oil filter in. It seems to be after I changed my gas mileage got worse! I was getting 17 to 18 mpg on the highway but now im getting 14 to 15. Well what the heck? Any advice would be wonderful! Should I do another oil change or what? Oh yes a second issue. when I press the gas pedal down at this same spot it makes an annoying high pitch squeeling noise no matter what speed im going its right when I hit this one spot about a quarter of the way down. Im guessing the throttle body? Anyone have a problem like this or any advice? Thanks for your help on both issues!!!

 

The synthetic is not the cause for your decreased mileage unless you used a thicker viscosity or out of spec. oil then possibly, but that is a big possibly. Not sure about the squealing though I've never experienced something like that nor have I heard of someone else having that issue.

 

I highly doubt synthetic oil caused such a decrease in MPG also

But are you calculating by hand or OHD? There are SO MANY things that can affect MPG. Outside temps, air filters, cross or head winds, hilly roads, low tire pressure, spark plugs and wires, ECT.

As for the squealing, is it at certain speeds or RPMs?

Can you lift the hood and while in park, raise the RPMs to duplicate it? It is likely a belt slipping at a certain RPM or one of the accessories. Get someone outside with you and try to find the problem. Or take the belt off completely and momentarily and see if the issue goes away. That will help narrow it down.

 

So what oil do you recommend running? Weight is my biggest question.

 

5w-20 amsoil here...no problems, same for the 4.7

 

Yeah with MDS you need to use 5w-20

 

I use 5w30 in my '04 4.7 per the manual. I believe Dodge changed it to 5w20 for the '08 though. If you've got a hemi with MDS then you need the 5w20 for the MDS to function properly if you do not I believe that pre MDS hemi's were speced for 5w30 as well, not sure about that though.

Point is, just use whatever weight the manual racommends, meets MS6395, is atleast SM rated, and atleast GF-4. Although come in I believe its November the new standard is going to be SN and GF-5.

 

Thanks for the input!

 

You're going to want 5W-30. I use Pennzoil Hi-Mileage in mine.

 

Changing to synthetic won't affect mileage. There is only one way to determine mileage, and it isn't via the computer. Squealing is usually the belt or a pulley.