Short answer: Buna-N (nitrile) is the economical choice for petroleum oils, greases and standard fuels below ~250°F. Viton (FKM) costs more but survives aggressive chemicals, ethanol-blended and modern fuels, and continuous heat to ~400°F. If the washer sees high heat plus harsh fluids, spend on Viton; for routine oil and fuel duty, nitrile does the job for less.
Buna vs Viton: side-by-side
| Property | Buna-N (Nitrile) | Viton (FKM) |
|---|---|---|
| Temperature range | −40°F to 257°F | −15°F to 400°F |
| Petroleum oil & grease | Excellent | Excellent |
| Gasoline/diesel | Good | Excellent |
| Ethanol blends (E15/E85) | Fair — swells over time | Very good |
| Aggressive chemicals/solvents | Poor–fair | Excellent |
| Low-temperature flexibility | Better (to −40°F) | Stiffens below −15°F |
| Weather/UV/ozone | Poor | Good |
| Relative cost | $ | $$$ |
Fuel systems: which washer to use
For sealing automotive fuel systems, Viton is the safer specification: it resists modern ethanol-blended gasoline, biodiesel and fuel additives that gradually swell and soften nitrile. Nitrile remains acceptable for older vehicles on straight gasoline or diesel, fuel filters, and short-service applications — it's what many OEM fuel washers were historically. In cold climates note the flip side: nitrile stays flexible to −40°F while Viton stiffens below about −15°F.
Oil and hydraulic service
Both excel in petroleum oils. Under 250°F, nitrile is the standard — crankcase drain plugs, hydraulic fittings, compressor housings, gearbox covers. Above 250°F, or near exhaust/turbo heat, Viton's 400°F ceiling earns its price.
Chemical service
Viton is the chemical-duty rubber: strong against fuels, solvents, many acids and aggressive process fluids. Two shared blind spots — neither Viton nor nitrile suits ketones (acetone/MEK) or hot steam service; for steam and water chemistry use EPDM instead (see neoprene vs EPDM).
When to choose each
Choose nitrile rubber washers for: engine oil, hydraulic fluid, grease fittings, straight gasoline/diesel, cold-climate equipment, budget-driven volume work. Choose Viton rubber washers for: ethanol-blend fuel systems, chemical processing, high-heat sealing near 250–400°F, solvent exposure, aerospace-grade reliability.
Shop both in every OD, ID and thickness: nitrile rubber washers · Viton rubber washers. Deeper reading: nitrile material guide, Viton material guide, automotive washer guide.
FAQ
Is Viton always better than Buna?
No — Viton is better in heat and chemicals, but nitrile is better below −15°F, costs a fraction as much, and matches Viton in plain petroleum oil service. "Better" depends on the fluid and temperature.
What rubber washers are best suited for sealing automotive fuel systems?
Viton for modern ethanol-blended fuels and long service life; nitrile is acceptable for straight gasoline/diesel and short-term or budget applications.
Can I use nitrile washers outdoors?
Not in sunlight — nitrile has poor UV and ozone resistance and cracks within seasons. Outdoors, use EPDM or neoprene unless oil resistance forces Viton.