| Property | NBR | FKM |
|---|---|---|
| Temperature range | −30°C to +100°C (short-term +120°C) | −20°C to +200°C |
| Mineral oil / hydraulic fluid | Excellent | Excellent |
| Petroleum fuel (non-oxygenated) | Good | Excellent |
| Oxygenated fuel (E10, E85, biodiesel) | Poor | Good–Excellent (Viton F) |
| Water / steam | Good (water); Poor (steam) | Fair (water); Poor (steam) |
| Acids / caustics | Poor | Fair–Good (concentration dependent) |
| Ketones / brake fluid | Poor | Poor |
| Ozone / weathering | Poor | Excellent |
| Cost | Low (baseline) | 3–8× higher than NBR |
| Typical hardness | 70 Shore A, 90 Shore A | 75 Shore A, 90 Shore A |
Use NBR as your default for mineral oil, petroleum fuel and general hydraulic service where temperature stays below 100°C. Switch to FKM when the fluid list includes synthetic lubricants, oxygenated fuels, higher temperatures (above 100°C continuous), or where NBR shows swelling or surface cracking in service. Do not use FKM in brake fluid, ketones or amines — these attack FKM as badly as they attack NBR.
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