Originally Posted By: Shannow
Originally Posted By: deoxy4
In a nut shell:
1. Lowering the viscosity of an oil a grade could result in a fuel increase of 1-2%. Let's say for example, running a 5W-20 oil will result in a 1-2% increase over a 5W-30 from the same manufacturer and type of oil.
Here's a chart from one of the oil development task forces...You can compare the relevant grades in question to get a feel for the relative shift between grades.
Originally Posted By: deoxy4
2. Running a synthetic oil could result in a 3-5% increase in fuel economy over a dino oil of the same viscosity.
It's been positied by one BITOG member that you would see 4% between a "regular" 0W20 and TGMO, but claims like that WITHIN a grade are pretty well bunk.
This, plus the 0.6% figure from Ford pretty much sums it up IMHO. Is there a MPG benefit with a thinner oil? Yes. But going from a 5w-30 to a 5w-20 is not going to give you a noticeable gain, nor one that would be measurable by your average consumer. It is measurable by the OEM under controlled conditions and is subsequently relevant to them for CAFE reasons as noted in the remarks cited from Ford.
To put this in perspective, going by Ford's number, if you had a car that got 30Mpg on 5w-30, switching to 5w-20 will yield 30.18Mpg. The consumer does not have equipment sensitive enough to measure that sort of gain and factoring in the variability in your service station fuel pumps, driving style, tire inflation, ambient temperature, humidity....etc it is impossible to correlate viscosity with economy.
For the consumer things like tire pressure and driving style are far bigger inputs to observed economy than lubricant selection. For the OEM, who has already go those things sorted on their side because their test conditions are controlled, chasing viscosity for CAFE makes sense.