Consideration of all requirements an oil must meet, or holistic formulation development, is the only way to ensure that, in the end, an oil is robust enough to meet cleanliness and durability requirements for ILSAC
and dexos1[TM] specifications.
FEI Sum is the summation of FEI1 and FEI2 and is one of pass/fail criteria for the ILSAC and API SN/RC specifications.
Although API SN covers a wide variety of SAE viscosity grades including mono-grades, ILSAC GF-5 specifies only 0W, 5W, and 10W multigrade oils.
However, it took almost 2 decades until one of Japanese OEMs developed it under ILSAC and API specifications .
The ILSAC GF-5 specification was not designed to incorporate new viscosity grades in the middle of its period.
As the next generation of ILSAC specification, ILSAC GF-6 development has started and GF-6B category has been proposed for viscosity grades lower than SAE 0W-20.