In anticipation of a new heavy-duty engine oil category due out in 2016, which will feature a lower-viscosity sub-category for improved fuel economy in newer engines, Shell has been running a fleet field test comparing Shell Rotella 15w-40, 10w-30, and an experimental 10w-30 with viscosity properties similar to what is expected in the new category.

This week, it laid out the parts for inspection in the Clarke Power shop in Greensboro, N.C., and dared trucking journalists to tell the difference.

They couldn't. Even those who picked the "right" table said it was simply a guess, or read it from the body language of one of the Shell representatives.

While the new oil category, currently termed PC-11, will offer improvements in areas such as oxidation stability, aeration performance, scuffing/adhesive wear and shear stability, one of the biggest changes is the viscosity change.

Traditional SAE viscosity ratings have been measured in a "non-shear" environment at 100 degrees C (212 F). The new oils will be measured at 150 degrees C (302 F) and in a "high-shear" environment. (Shear is mechanical stress that causes the oil to break down at a molecular level, "shearing" the polymers that give the oil its multi viscosity rating.)

"One of the key things is the oil has got to be able to protect with thinner films and hotter temperatures, and no compromises in the durability or life of the engine," said Dan Arcy, global OEM technical manager. "Our customers have become extremely accustomed to getting long lives out of their engines and they're not going to be willing to give that up in exchange for fuel economy."

While the category has not been finalized, Arcy, who is heading up one of the subcommittees developing the category standards, said it has been recommended to be 2.9 cP to 3.2 cP (a measure of high temp/high shear performance). In comparison, a 15w-40 oil is about 4.2 cP.

Arcy said Shell already has demonstrated that you can get 1.6% better fuel economy in on-highway trucks by going from that 4.2 to a 3.5, which is currently available in lower-viscosity engine oils. In fact 3.5 is the limit of CJ-4 oils. There is more fuel economy to be had by dropping lower, he said, although he couldn't give an estimate of just how much more.

Matt Urbanak, lead formulator, works closely with the field trial team and notes that Shell currently has more than 200 vehicles on test with an annual mileage of more than 25 million miles.

"It's normal to see wear" in an engine teardown, he explained. "What we're looking for is whether there's anything abnormal about that wear."

The engines torn down for this inspection were 2011 model year Detroit DD15 engines with about 550,000 miles on them. In Freightliner Cascadia trucks, they've been working in a North Carolina fleet driving in team operations coast to coast about 250,000 miles a year with average loads around 76,000 pounds. The recommended oil drain is 50,000 miles and this fleet average was a little higher, about 55,000 miles. Oil analysis performed at oil changes and at the mid-point of the change interval indicated the low-viscosity oil was performing as well as the traditional 15w-40 oil.

Howard Hill, engineer, lubricant technology, who's in charge of the field test program, said, "The comparison we're getting between the 15w-40s, the 10w-30s and new high-temp/high shear, you can go around and look at these parts, there is not a difference – there is absolutely not a difference. It bears out the chemical analysis. The iron wear rates, I think the highest was about 100 ppm, and Detroit Diesel's condemnation level is 250 ppm, so we're well under that condemnation level. We like how the oils are performing. We're very, very pleased with the new technology, and it's proven itself out with the engine inspections."

Below, watch Hill explain the findings: