Why 10w30?

[kschachn]

Manufacturers want the engine (and the oil) to warm up as fast as possible to reduce emissions. A thicker oil will warm up faster than a thinner one.

 

[SR5]

Once the oil is warmed up in the highway there's virtually no mpg improvement from a 10w-30 to a 0w-16 unless you have a very high mpg car to extrapolate the difference. But in the first 10-15 minutes of city driving yeah much bigger difference in mpg and that's what I think the manufacturers are getting at.

I agree, most of the fuel economy gains with a thin oil come during the early warm up phase. If you are a short tripper in a cold climate then 0W16 and 0W20 makes sense to me.

 

[kschachn]

I agree, most of the fuel economy gains with a thin oil come during the early warm up phase. If you are a short tripper in a cold climate then 0W16 and 0W20 makes sense to me.

The long-term difference is at operating temperature. Warmup is a transitory and relatively brief period, and all oils are very thick at low temperatures. Besides as I noted above what the automakers want is for the engine to reach operating temperature as quickly as possible. An oil with a 0W winter rating is not guaranteed to be thinner until the temperature reaches -35F or so, above that temperature it's anyone's guess.

 

[kschachn]

Once the oil is warmed up in the highway there's virtually no mpg improvement from a 10w-30 to a 0w-16 unless you have a very high mpg car to extrapolate the difference. But in the first 10-15 minutes of city driving yeah much bigger difference in mpg and that's what I think the manufacturers are getting at.

That's simply not true. You should read up on the ASTM fuel economy tests that result in the official MPG reading for an oil which is reported to the EPA for CAFE credits.

If that were true then manufacturers would specify 0W-40 rather than 0W-20 grades.

 

[SR5]

I could be wrong, but I thought the standard fuel economy test included a warm up phase which could have a significant impact on your overall official figure.

From before, Joe the oil formulator said:
I'd opt for the A3/B4 oil because it HASN'T been put through the US fuel economy tests. I don't like these tests because as often as not, they 'encourage' formulators to drop the oil's CCS viscosity to silly levels to cheat the test. Lower CCS means more VII & higher Noack.

 

[kschachn]

The ASTM tests for fuel economy IRT motor oils is over a hundred hours long, it includes aging and other tasks. The engine is tested at many loads and speeds, any warmup intervals are as minor as it is in real-world driving.

 

[JavierH19]

I agree, most of the fuel economy gains with a thin oil come during the early warm up phase. If you are a short tripper in a cold climate then 0W16 and 0W20 makes sense to me.

true but only really so in the winter. in the 86-88f texas mornings 15w-40 wouldn't struggle and id get 17mpg immediately cruising at 45 straight out of my neighborhood after the engine only ran for 2 minutes. It's only when its very very cold that it makes sense so a 5w-30/40 is honestly just a great all rounder since its plenty thin when below 20f but decently thick when warmed up.