Transmission oil seal assemblies in heavy-duty commercial vehicles—such as buses, garbage trucks, and dump trucks—are continually immersed in harsh, automatic transmission fluid and must operate over a temperature range from -40°F to +264°F (-40°C to +129°C). To enable success in such applications, Minnesota Rubber and Plastics modified an oil seal assembly design using a high performance plastic polyetherimide (PEI) and a fluoroelastomer (FKM) o-ring. Besides challenges such as long-term strength requirements and the ability to seal correctly in a harsh operating environment, the assembly had to withstand transmission oil exposure under alternating pressures for thousand of cycles. By choosing the new materials, the company was able to meet the necessary temperature, fluid, and wear resistance requirments along with close tolerance sealing features. The new seal assembly design also is more cost effective than the previous design using traditional materials.