Originally Posted By: Panzerman
That is purely a Hypothetical response figuring thinner oil takes less energy=Less fuel. Real world, it doesn't work that way. I challenge you to use a thicker oil instead of a light weight oil specified and find out it makes no difference, then make that assumption. I have found-Real World-No difference between 5w20/5w30 or 10w30 or 0w40 now.
It is there, and it is real.
In standardised tests, the thinner oil uses less fuel ... to do the job of the standardised tests... they are standardised to take the random driver factor out of it.
In the standardised testing, to achieve a GF-5, the oils MUST produce repeatable economy gains over the reference oils, and then maintain some smaller fraction of that over an extended period.
And it's of the order of 2% overall...still there and still a fact.
But drivers aren't standardised, and you aren't driving the standardised warm-up and load cycle...so you firstly get different numbers from the EPA figures. And secondly, the chances of you ever getting repeatable results to 1-2% accuracy are nil.
I DID use a thicker oil in my J Car.
went from an SAE 30 to a 25W70...book end extremes, and got a serious mileage penalty, and a very sluggish engine when cold (2.0L J-Car wagon could scarcely get out of it's own way at the best of times.).