Darin -
I'm not doubting that there could be something about those oils/lubes that could return better fuel economy, but looking at the sites you posted, they don't really appear to be "claims" when read in context, so much as suggestions. They're also not "en comparo" to anything else.
For example - on the Delvac site, there is a table which lists features in one column, and their "advantages and
potential benefits". Potential, sure. Actual? We dunno.
Later on the site, they suggest that you use it in tandem with some other fluid/additive for max fuel economy/wear/life:
Quote:
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Originally Posted by delvac site
Use as a companion with Mobil Delvac Synthetic Transmission Fluid 50 in transmissions for maximum drive train life, efficiency and fuel economy improvement
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Clever.
The Castrol site says:
Quote:
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Originally Posted by castrol site
Synthetic based to maximize oxidative stability and fuel economy
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So it's not really a claim so much as saying that "we're really trying". Again, with nothing in comparison, which means the claim
could be fulfilled just by changing your old fluid, which will obviously give you an FE boost simply because it's worn out.
Unfortunately, I'm not going to be able to test it, either, on my F150, because any gains I see could be attributed to having changed the fluid that's been in the truck since 11/95 when it came off the assembly lines.
So, in closing, while I don't doubt that there is something about some synthetic lubes that can improve fuel economy, I, in good conscience, can't say for sure, because I have yet to see a convincing piece of data which shows an improvement and couldn't be attributed to some other variable.