There are a lot of over the road trucks with several hundred thousand miles on a single oil change. They use bypass oil filters that filter to less than a micron.
If you are reducing friction, FE has to go up! How can it not? If nothing else your vehicle will coast farther!
On my Tahoe, I changed the transfer case and both differentials to AMSOIL and got almost 100 miles per tank. The engine and transmission gave an additional 38 miles per tank. On my wife's 99 GMC 4x4 pickup changing the transfer case and both differentials to AMSOIL got 74 more miles per tank. She didn't get as much as me because she only has a 25 gallon tank and GM put a semi-synthetic in the rear differential. I still need to change her engine and transmission to AMSOIL. Till I install my bypass oil filter, i change my oil once a year and the filter twice.
Kevin A Thornton
For Speed Equipment, Nitrous Express firstname.lastname@example.org
After changing oil it seems like I notice that the cars I drive run smoother. I have no way to quantify how my car runs post-synthetic oil, but my car seems to glide/coast better down hill, could also be I put the tire psi to 36 well.
I guess I rather trust the wear test that Amsoil, and I suspect other oil compenys refernce, and use to compair their oils to other oils, and I figure that if a more uniform oil provides better wear protection, and less brake down, alowing longer engine life, even without increased gas mileage, then it's worth it, the first time I used synthetic oil I wasn't really looking for better mileage, I thought that 38mpg out of a 20 year old car was good, but the car was in good shape and I wanted it to keep running for ever, replacing engines or cars as a whole adds up, so is there a draw back of useing synthetic oil? in my expearince yes, it will find leaks faster if they are already there, neither of my motorcycles, nor my civic leak a drop, my crx burns it maybe 1/3rd faster then dino oil, and with highway driving that is a quart and a half per tank (why it's waiting for me to swap to a lower mileage engine)