Most elastomer selection charts and application literature relating to elastomer usage recommend that ethylene propylene (EP) not be used in contact with petroleum fluids. However, when properly applied, EP rubber will successfully seal petroleum fluids in many diesel engine applications. The reason for this discrepancy is that industry generally equates“fluid resistance” (which really should be “resistance to swelling in fluid”) to resistance to deterioration in fluid. An elastomer may be swelled by a fluid, and not be chemically deteriorated by that fluid, and conversely an elastomer may not be swelled by a fluid but be degraded by that fluid. This confusion could be eliminated if both fluid swelling and fluid degradation information were clearly reported.Successful applications of EP for static seals are described, design considerations are discussed, and a swell pressure measurement device is described.