This paper presents the data from an SAE F&L 2 investigation into the viscosity increase at −18°C of crankcase oils as a function of mileage, engine type, type of service, viscosity grade and base oil type (petroleum or synthetic). The total data base from the survey is 1,530 viscosity measurements. These have been classified to show the viscosity increase for each of the major viscosity grades. The data show that 50% of all SAE 5W-X and SAE 10W-X oils go out of grade by 2,500 miles of use. SAE 15W-X and SAE 20W-X oils show better stability and remain in their respective low temperature viscosity grades for approximately 11,000 miles. While this survey has quantified the amount of viscosity increase in typical passenger car service, it does not provide any data to show the effects of this viscosity increase on startability in the field. A second survey is currently underway to cover this and the results will be published in a future paper.