"My laboratory provides feedback on kinematic viscosity at 100 degrees C for our engine lubes. At what level of viscosity change should we drain the engine oil?"
Because a change in viscosity, whether an increase or decrease (or both, therefore negating any perceivable change), can be an indicator of a number of problems, monitoring viscosity alone as an indicator of engine oil change is insufficient, especially on engines.
Engines can suffer fuel dilution resulting in a decrease in viscosity, as well as glycol or soot loading resulting in an increase in viscosity. Changing the oil will not cure the root cause of these problems.
In addition, if a multigrade engine oil is used, then a decrease in viscosity is possible because the viscosity index improvers shear during service. Further inspection or a more detailed confirmation analysis will be necessary to determine the cause if the viscosity changes before the end of the anticipated service life.
However, on engines, setting both upper and lower caution and critical limits will be an alert to oil life and contamination issues. The recommended lower limits are -5 percent and -10 percent, while the upper limits are +10 percent and +20 percent. These kinematic values are based on changes in centistokes at 100 degrees C.