I know of only this study using taxis as the main experiment. I am going to review it here. Las Vegas taxi service was considered one of the most demanding environments for an engine lubricant in North America by OEM’s.
Evaluation of SAE 0W20 GF-4 Prototype Formulation in Severe Taxi Fleet Service, Damen, Broom, Hartley and Riley.
My preface: Over the years many have referred to “The Taxi Study” as a reference that HTHS levels less than say 3.5 will result in accelerated wear. No oil should be used if the HTHS is less than 3.5... Engines will not last 100,000 miles as CAFE is mandating the use of thin oils with HTHS of less than 2.7... The study is proof that they only care about CAFE and no longer about engine longevity... Forget about the probability that replacing a car every 100,000 miles will have a greater impact on the environment than 0.5 better MPG in your car or truck. And the list goes on.
The study used 4 vehicles running 0W20 and 4 using 5W20 experimental oils from Infineum USA. The oils were group II/II+ mineral based formulations. Each car was driven for 100,000 miles. They each had 10,000 mile oil change intervals in what was considered the most severe service, taxis with extended idle times, mostly start/stop service, hot 108F heat, and 3 times the recommended oil change times. Yellow Checker Star Cab service ran the 8 Ford Windstar 3.8L vehicles. The time was 12-18 months of severe service - a lot of stressful miles in such a short time if you ask me.
The oils: 100C viscosity 8.7-8.9, HTHS 2.6, Noack 13.3-14.9, ZDDP 0.05. Tests were engine tear downs, oil analysis and bench tests.
Results:
Iron levels were thought to be acceptable if below 100ppm and actually averaged 70 during the 10k intervals. Lead was 7, aluminum was 6, and copper was 22. The acceptable limits were 40 for lead, 50 for aluminum and 50 for copper. Remember that the oil change interval was 3 times longer than recommended!
Viscosities despite having early thinning actually exhibited oxidative thickening up to a viscosity of 10 at 100C (a 30 grade oil) at 10,000 miles. The HTHS also thickened to as high as 3.5. Certainly engines experienced both ends of the scale.
The fuel economy was statistically significant comparing the 0W to 5W oils with a 0.2 MPG difference. (Some say you should use a 5W over a 0W oil and better yet a 10W for wear reduction but that was not evident here).
Overall engine tear down results: Testing bearings, sludge, varnish, valve trains revealed “excellent” results. Lab and bench tests suggested “excellent” wear performance.
My conclusions.
I see no evidence “proving” that wear would be “excessive” if the HTHS was below 3.5.
Wear with the 20 grade oils, in the most severe conditions, was certainly not excessive. At 100,000 miles engines were nowhere near end of life.
If we were to use current 20 grade oils made from better base stocks and better additive packages one might expect even less wear.
ali
Evaluation of SAE 0W20 GF-4 Prototype Formulation in Severe Taxi Fleet Service, Damen, Broom, Hartley and Riley.
My preface: Over the years many have referred to “The Taxi Study” as a reference that HTHS levels less than say 3.5 will result in accelerated wear. No oil should be used if the HTHS is less than 3.5... Engines will not last 100,000 miles as CAFE is mandating the use of thin oils with HTHS of less than 2.7... The study is proof that they only care about CAFE and no longer about engine longevity... Forget about the probability that replacing a car every 100,000 miles will have a greater impact on the environment than 0.5 better MPG in your car or truck. And the list goes on.
The study used 4 vehicles running 0W20 and 4 using 5W20 experimental oils from Infineum USA. The oils were group II/II+ mineral based formulations. Each car was driven for 100,000 miles. They each had 10,000 mile oil change intervals in what was considered the most severe service, taxis with extended idle times, mostly start/stop service, hot 108F heat, and 3 times the recommended oil change times. Yellow Checker Star Cab service ran the 8 Ford Windstar 3.8L vehicles. The time was 12-18 months of severe service - a lot of stressful miles in such a short time if you ask me.
The oils: 100C viscosity 8.7-8.9, HTHS 2.6, Noack 13.3-14.9, ZDDP 0.05. Tests were engine tear downs, oil analysis and bench tests.
Results:
Iron levels were thought to be acceptable if below 100ppm and actually averaged 70 during the 10k intervals. Lead was 7, aluminum was 6, and copper was 22. The acceptable limits were 40 for lead, 50 for aluminum and 50 for copper. Remember that the oil change interval was 3 times longer than recommended!
Viscosities despite having early thinning actually exhibited oxidative thickening up to a viscosity of 10 at 100C (a 30 grade oil) at 10,000 miles. The HTHS also thickened to as high as 3.5. Certainly engines experienced both ends of the scale.
The fuel economy was statistically significant comparing the 0W to 5W oils with a 0.2 MPG difference. (Some say you should use a 5W over a 0W oil and better yet a 10W for wear reduction but that was not evident here).
Overall engine tear down results: Testing bearings, sludge, varnish, valve trains revealed “excellent” results. Lab and bench tests suggested “excellent” wear performance.
My conclusions.
I see no evidence “proving” that wear would be “excessive” if the HTHS was below 3.5.
Wear with the 20 grade oils, in the most severe conditions, was certainly not excessive. At 100,000 miles engines were nowhere near end of life.
If we were to use current 20 grade oils made from better base stocks and better additive packages one might expect even less wear.
ali