http://www.fleetowner.com/lubricants/new-generations-oils-begin-take-holdExcerpts...Thinner but just as durable
Several years of industry collaboration yielded two variations of the next generation of oil—CK-4 and FA-4.
“The oils are thinner but just as durable and are able to withstand a high-shear engine environment,” said Brian Humphrey, OEM technical liaison at Petro-Canada Lubricants. Higher fuel economy “can be achieved because lower viscosity oil means less friction in the engine, leading to a reduction in both fuel consumption and [carbon dioxide] emissions—while still offering exceptional levels of wear protection.”Back Spec'ng
"API’s Ferrick said while Detroit Diesel recommended use of FA-4 in its latest engines from the start, other manufacturers are taking a more cautious approach, and are “looking to get a little more assurance FA-4 is suitable.”
Some of the hesitation involves the limited backwards compatibility of FA-4, because some older engines are not designed for low viscosity oils.
The latest engines have slightly more finely polished surfaces on critical parts, according to Valvoline. These surfaces can tolerate the slightly thinner film of oil produced by FA-4 because of its lower viscosity.
“Unless a fleet has all [Detroit Diesel] series engines, they really are kind of handcuffed,” said Paul Cigala, applications engineer for commercial vehicles at ExxonMobil"Mixing FA-4 with CK-4
“Take great care not to mix FA-4 with CK-4. Co-mingling will adversely impact the viscosity grade of the FA-4 oil, and that is part of the reason they are getting FA-4 oil in the first place,” he said.