By Tom Schaefer
"The largest effect motor oils have on fuel economy is through viscosity, followed by friction modification. Lower viscosity oils can save a few percent in fuel consumption when measured by industry standard engine tests, regardless of whether the base oil is synthetic or mineral. Friction modifiers will kick in a little more, but the combined effect is rarely more than 3% compared to 10W-30s or 40s.
Synthetics can play a role in fuel economy in three ways:
1. Synthetics are generally less volatile and have higher VIs than mineral oils, so you can make thinner oils without sacrificing oil consumption, catalyst damage, and HTHS viscosity. In other words, you can make better low viscosity motor oils from synthetic base oils than from mineral base oils.
2. Synthetics are generally more oxidatively stable than mineral oils, so the oil can retain its lower viscosity longer. This means the favorable effect from lower viscosity may last longer and therefore save more fuel over time.
3. Some synthetics utilize polar base oils that can lower friction and improve lubricity. This makes them behave like a friction modifier, although they may compete to some extent with friction modifier additives and therefore not add much.
As for measuring the fuel efficiency, only carefully controlled engines tests such as those used for GF-4 certification are sensitive and repeatable enough to see the small contribution oils provide. The average consumer has far too many variables to reliably measure or even see oil related fuel savings, such as city/highway mix, road conditions, tire pressure, driving style, engine condition, fuel brand and dispensing accuracy, sensor condition, temperature, wind, humidity, etc. I plotted my MPG for 13 years on the same car and same gas, and the results varied from 16 to 29 MPG, with 95% of the data being +-15% from the mean. Imagine trying to see a 2% improvement in a plot that swings that wildly. I have to giggle when I hear people report a 5 or 10% improvement after one tank of gas. Such testimonials are worthless.
Bottom line, you will never know exactly how much gas you are saving from your motor oil, nonetheless the contribution from synthetics, but you can be sure that lower viscosity oils will save some fuel and a good synthetic a little more." - Work Cited: Tom Schaefer
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