Why is my 15W40 engine oil dropping into the SAE 30 grade on my report even though the lab is not detecting any fuel dilution?

Multigrade oils (i.e. 5W30, 15W40, 20W50) contain viscosity index (VI) improvers. The purpose of these additives is to keep the oil’s viscosity within range as temperatures increase. Depending on clearances, speed, oil temperature and additive chemistry, the molecular VI improvers can “shear” in service. This causes the oil viscosity to decrease. A very small change in viscosity can cause the sample to cross the boundary between one SAE grade and the next. When you see your oil SAE grade decrease from SAE 40 to SAE 30, for example, look at the actual change in the viscosity test result. This will tell you the severity of the change independent of the indexed SAE grade.