Standardized vehicular On-Board Diagnostics (OBD) systems offer access to information commonly used for fault notification and reactive diagnostic services. Recently, there have been efforts to use OBD data to diagnose and predict faults prior to catastrophic failure events. Engine oil service life, a parameter directly related to engine longevity, is difficult to measure conventionally. We show that the rate of engine coolant temperature rise, readily obtainable through the OBD suite, can serve as a proxy to indicate the remaining engine oil life. We demonstrate consistent results for one vehicle under similar environmental and unloaded engine operating conditions. We also examine the validity of this approach under varying environmental and engine loading conditions with tests on a second vehicle.