By using low-grade seals, you can end up with premature wear or hardening of the seals, allowing large quantities of oil to enter the valve guide. Today's smaller engines are expected to do more work per pound than their larger predecessors, which results in higher operating temperatures. Rubber valve stem seals composed of different compounds vary tremendously in physical properties. Two parts may look and feel identical when new, yet one will last many times longer than the other. If the rubber compound doesn't have the right properties for the particular application, it can wear, harden, split, crack or break into pieces.
Nitrile works well in older engines, except certain high-performance applications.
It is relatively inexpensive, but if used as a substitute when premium grade materials are specified, it can show signs of deterioration and failure in a matter of months. Polyacrylate compounds have much higher temperature resistance, and present a good balance between heat and oil resistance and price. Viton is currently the best material available for today's engine designs, particularly the smaller import and domestic models. The material specified by the engine manufacturer can be expected to give satisfactory service. Some parts suppliers do not always observe these standards.