Originally Posted by
AlphaMicra
does he think that a full synthetic would make fuel economy appreciably better?
If you did a LOT of very short trips / cold starts in freezing weather, you would see a tiny improvement in fuel economy by using the same grade of synth vs. conventional oil. For the same reason Pete mentioned: synth flows much more easily at freezing temps, so there's less internal resistance. But once the engine is up to normal operating temperature, there's little difference.
So the average person won't see any fuel savings by switching.
However you're right: switching to the lowest recommended viscosity for a particular vehicle may mean going to a synthetic oil (eg. you can't get 0w20 in a conventional oil, and some cars allow it), and that saves a small amount of fuel.
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View my fuel log 2015 Micra S manual: 5.0 L/100 km ... 56.5 mpg (Imp) ... 20.0 km/L ... 47.0 mpg (US) ...