Synthetic rubber (elastomeric) o-ring seals are widely used in modern CI and SI power systems. They are increasingly challenged by higher thermal inputs, design considerations, and long-term warranty issues. This paper presents comprehensive, quantitative aging data for evaluating popular elastomers used for sealing coolants. Three categories of commercial heavy-duty (HD) engine coolants were used as test media. The commercially available elastomers tested in this study were: fluoroelastomers (FKM-type 2), tetrafluoroethylene–propylene rubber (FEPM), and hydrogenated nitrile rubber (HNBR). The study sought to duplicate the effects of multiple engine startups and shutdowns. The test protocol involved 100 twenty-four hour cycles consisting of 16 h operation at 150°C and an 8-h cool down. The test fixtures, run in triplicate, provide O-rings sealing axially (flange seal design), radially (piston seal design), and a straight thread port seal. The test fixtures subjected seals to saturated vapor on one side and liquid on the other. Data was developed and reported in accordance with ASTM D1414-94 and ASTM D412-06a. Compressive stress relaxation is reported in accordance with ASTM D6147. To monitor degradation of the fluoroelastomers, coolant was monitored for fluoride ion activity. A brief discussion on polymer stability in high pH aqueous and nonaqueous fluids is included.