My 2003 4.2 S-type has about 72,000 miles on it so I figured it was time to change the differntial fluid. I got out about +1.2 quarts. It looked used but not too bad. I got a good deal on Royal Purple 75w-140 at Advance ($10.50 with coupon). Just so you guys don't think I am one of those indorsement guys, I have tried the Royal Purple 5w-30 oil and it is over priced for what it is. With that said, I loaded up my wife and two kids and went on a 120 mile road trip. We stopped at an outlet mall and loaded up the trunk. We averaged 32.0 mpg. I usually get about 25-28 mpg on the highway if I'm lucky.

Should this stuff influence my mpg's that much? I did not hyper mile or anything, just set the cruise at about 65 mph and used 92 octaine gas from Thorton.

We will see how things go but for now I'm really impressed with this stuff.

I took a couple of pics when we got off the highway.

Mmmmmmmm.

Fresh oil is always going to reduce friction when compared to what came out, so maybe your oil was more "off" than it appeared.

I just did "her" S type 3ltr, and what came out was quite disgusting for a sealed unit, so Nulon Synthetic went back in, and YES it appears to my OLD ears, to be quieter. The next big run of about 5000kms at Christmas will tell all.

I am a FIRM believer of changing these fluids every 2 years, and I have never had a diff/trans/power steer pump failure.

Did you check that MPG with a calculation from miles traveled and gallons for fill up? Cause if it did match, I am changing my diff fluid!

That's pretty impressive. It was a long trip but still only one trip, so an update in a week or so might be a good thing.

I did not check it yet. I wanted to drive it for a week or two, start fresh and then do the math. As far as the spedo doing the math for the MPG's, it has always been accurate.

From the physics point of view, the increase in mileage cannot have come from a change in lubricating oil. Given that an engine is producing power at a constant rate, if the old oil was too thick causing excessive friction, the energy still has to go somewhere since it can't get to the wheels. No different than the brakes, the energy would be turned into heat and radiated into the atmosphere.

The amount of energy required to drop mileage by 4-7 mpg on a continual basis would produce enough heat to melt the back end of the car.

I commonly see variations in mileage along the the lines of your observations with no changes made to the car at all.

I changed the rear diff oil on my 2005 STR @65K mile with no change in noise or mileage. It was pretty dirty like yours but not alarming.
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Where did you get the coupon?

I was looking at the right place at the right time.

Advance Auto Parts Coupons & Promo Codes 2012, Advance Auto Parts Online Discount Codes

It was the first one on the top of the page.