Just trying to quantify things still...
If the purple vertical line here represents a 1% increase in fuel economy (per the test linked above), then the blue vertical line has to represent around a 3.5% increase in fuel economy. That is only at 60C / 140F. As the temperature drops, the difference in viscosity increases to quite a bit more. Our cSt at operating temps is about 13. An increase of 5 reduces fuel economy by 1%. Obviously it can't be completely linear, but the cSt at 0C / 32F is over 1400 for 10W30 oil. So, I think its safe to say that heating the oil up must have some pretty decent fuel economy benefits.
