I recently changed the oil on my Subaru to Pennzoil Platinum 0W-20 Advanced Fuel Economy, from PP 5W-30. At the end of my first drive with the 0W-20 I noticed that the MAF reading on my datalogger at warm idle was under 2.0 g/s, which was unusual. I decided to look at some recent datalogs I had taken with the old 5W-30 to compare fuel consumption.
I compared datalogs taken at warm idle since 80-90% of engine load at idle is due to friction of oiled components, so any effect would be more noticeable. Engine sensor readings relevant to fuel economy were monitored to make sure the tests were comparable, most importantly coolant and oil temperature, air fuel ratio, ignition timing, rpm, and a/c compressor status. All but one of the tests were done on the same tank of fuel. The oil filter wasn't changed, and no other engine maintenance was done recently.
Here are the results:
With the 5W-30, fuel consumption at idle was 6.5% higher on average. The difference would be lower with the engine under load. I estimate a 1.5% improvement in part load cruising, and 0.8% improvement at WOT, based on some data from engine friction studies. I'd expect larger differences with a cold engine.
The difference is a lot higher than I would've expected, since the old 5W-30 should have been close to the same viscosity as the fresh 0W-20, from fuel dilution and a bit of shear.
I suspect that most of the difference is due to oil formulation. The 5W-30 was Dexos1 Gen2 (Sept. 2020). The 0W-20 is Dexos1 Gen3 (June 2022), and it's a bit lower in detergents and ZDDP, and contains no boron. The "Advanced Fuel Economy" might not just be a marketing gimmick. The age of the 5W-30 shouldn't have had a large effect with only 6,600 km on it. The test done with 3,000 km on the oil showed a similar result. I'll take some more logs once the 0W-20 is fuel-diluted to see if it makes a difference.
I compared datalogs taken at warm idle since 80-90% of engine load at idle is due to friction of oiled components, so any effect would be more noticeable. Engine sensor readings relevant to fuel economy were monitored to make sure the tests were comparable, most importantly coolant and oil temperature, air fuel ratio, ignition timing, rpm, and a/c compressor status. All but one of the tests were done on the same tank of fuel. The oil filter wasn't changed, and no other engine maintenance was done recently.
Here are the results:
With the 5W-30, fuel consumption at idle was 6.5% higher on average. The difference would be lower with the engine under load. I estimate a 1.5% improvement in part load cruising, and 0.8% improvement at WOT, based on some data from engine friction studies. I'd expect larger differences with a cold engine.
The difference is a lot higher than I would've expected, since the old 5W-30 should have been close to the same viscosity as the fresh 0W-20, from fuel dilution and a bit of shear.
I suspect that most of the difference is due to oil formulation. The 5W-30 was Dexos1 Gen2 (Sept. 2020). The 0W-20 is Dexos1 Gen3 (June 2022), and it's a bit lower in detergents and ZDDP, and contains no boron. The "Advanced Fuel Economy" might not just be a marketing gimmick. The age of the 5W-30 shouldn't have had a large effect with only 6,600 km on it. The test done with 3,000 km on the oil showed a similar result. I'll take some more logs once the 0W-20 is fuel-diluted to see if it makes a difference.
