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Originally Posted by MetroMPG
The secondary advantage of synthetics is more stable viscosity at low temperatures. Which in a cold climate translates into easier starts & better efficiency but only while the engine is coming up to normal operating temps. The actual fuel economy gain in that scenario would be tiny, though.
It's worth noting that no major motor oil company (that I know of) claims "improved fuel economy" as one of the benefits of its synth motor oils relative to the same weight non-synth.
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Mobil 1, IIRC, basically says that their 0w30 is more stable than, but has close to the same viscosity, as 5w30, but does claim increased fuel economy, by up to 2%
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Helps to increase engine efficiency and improve fuel economy up to 2%, based on a comparison versus those grades most commonly used. Actual savings are dependent upon vehicle/engine type, outside temperature, driving conditions, and your current engine oil viscosity.
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M1, unfortunately, is the only producer I know of that makes 0 weight oils, so the comparison has to be made with 5 and 10 weight oils, which kind of skews the result. Then again, the engine only sees that weight when it's cold, and based on normal driving tests, it's only cold a few mins, so maybe the higher hot-engine stability attributes to the better FE, in which case, it's an even test with any hot-30 oil.