A taxi field test in 1999 resulted in unusually high used oil MRV TP-1 viscosity in the first 16,000-kilometer drain oil. A subsequent root cause investigation revealed that contamination of the test oil by carry-over of the factory-fill oil followed by oil aging in the vehicle was responsible for the unusual high MRV TP-1 viscosity. Contamination by the factory-fill oil alone cannot account for the high MRV TP-1 viscosity; oil aging in vehicles is an essential co-factor. While the precise mechanism has not been determined, high MRV TP-1 viscosity and yield stress appear to be the consequence of reduction in PPD effectiveness but not PPD degradation. However, the MRV TP-1 viscosity and yield stress of such used oil can be restored to acceptable levels by an optimized PPD system. The study found that used oil MRV TP-1 pumping viscosity and yield stress can be highly dependent on the viscosity index improvers used in the oils involved. However, oil robustness can be enhanced by a rigorous field test protocol.