FKM (fluoroelastomer, sold as Viton) sustains continuous service to roughly 204°C in hot oil and aggressive fuel streams, while standard NBR (nitrile, Buna-N) tops out around 100–120°C continuous before hardening and cracking become dominant failure modes [S1][S5].
The trade-off is cost and low-temperature flexibility: NBR is the most widely specified O-ring globally and runs at roughly 1/3.9 the raw material price of FKM, while FKM loses elasticity below about −17 to −25°C depending on compound [S1][S3]. For a deeper look at the fluoroelastomer family itself, the FKM polymer backbone and monomer trade-offs piece maps how fluorine content drives heat and fluid behaviour.
Temperature window: where each material lives
Standard NBR compounds are rated −30°C to +100°C in general service, with short-term peaks to +120°C; hydrogenated NBR (HNBR) and special NBR grades push the high side toward 125–135°C, but the polymer backbone still ages rapidly above that window [S1][S4][S5]. FKM continuous service extends to roughly +204°C with intermittent peaks to +316°C, while remaining serviceable down to about −17°C on general-purpose grades and −25°C on flexibility-optimised compounds [S1][S3][S5]. Above 120°C, FKM is the only realistic choice of the two for any sustained hot-oil or fuel exposure; the fluororubber reference captures this thermal envelope in more detail.
Hot oil, diesel and gasoline: the headline comparison
For petroleum-based media the picture is more nuanced than marketing suggests: both nitrile rubber and FKM O-rings resist mineral oil, hydraulic fluid, diesel and lubricating oil adequately, and NBR is the industry's default commodity seal in those fluids at moderate temperature [S1][S2][S5][S8]. The gap widens in two directions: at higher temperature NBR hardens, loses elongation and cracks, while FKM holds its mechanical properties; and with aggressive fuel chemistries, gasoline, biodiesel, oxygenated fuels, aromatics and synthetic lubricants, FKM keeps its volume swell and tensile strength where NBR swells excessively and degrades [S1][S5]. "Both NBR and FKM offer excellent resistance to oils and fuels" is true only inside the temperature and chemistry window where the O-ring is asked to perform [S5].
Side-by-side decision matrix
Using the data published in mid-2026 manufacturer guides, the four most decision-critical criteria line up as follows [S1][S2][S3][S5][S8]:
Continuous temperature range: NBR −30 to +100°C (peaks to +120°C) versus FKM −17 to +204°C (peaks to +316°C) [S1][S5].
Hot oil / fuel resistance: NBR is excellent for mineral oil, hydraulic fluid and diesel below 100°C; FKM is excellent for hot oil plus gasoline, biodiesel, aromatics and synthetic lubricants to its full temperature ceiling [S1][S5].
Low-temperature flexibility: NBR is serviceable to roughly −30°C, FKM general-purpose to about −17°C; this is one of the most common FKM selection mistakes in cold-start applications [S1].
Raw material cost: NBR is the cheapest common elastomer; FKM runs about 3.9x the NBR raw material price, with FFKM roughly 329x NBR [S3].
Chemistry, ozone and process-fluid edge cases
Outside petroleum fluids, the two materials diverge sharply. NBR performs poorly with ozone, UV, ketones, esters and chlorinated hydrocarbons, so any outdoor hydraulic skid, fuel system exposed to weathering, or process using aggressive solvents is a poor fit for nitrile regardless of temperature [S1][S2][S5]. FKM tolerates ozone, UV, acids, aromatic hydrocarbons and most industrial cleaning agents, but it still fails against ketones, amines, low-molecular-weight esters and Skydrol-type phosphate-ester hydraulic fluids, which limits its use in some aerospace and chemical-process duties [S1][S5]. For steam, hot water, glycol and brake-fluid service, neither FKM nor NBR is the right call: EPDM or specialty HNBR is the standard answer, as covered in the EPDM vs NBR vs FKM butterfly valve seat temperature limits comparison.
Dynamic vs static sealing, hardness and compound choice
Seal geometry changes the answer. NBR has higher tensile strength, abrasion resistance and tear resistance than FKM, which makes it the preferred elastomer for dynamic O-ring motion, reciprocating rods, pumps and any application with significant mechanical wear at moderate temperature [S1][S2][S8]. FKM has better compression-set recovery and dominates static hot-oil sealing, flange joints and engine gasket duty where the seal sits still but the fluid is hot and aggressive [S1][S5]. Hardness matters as well: 70 Shore A is the commodity default (NBR B1000, B1002; FKM Viton-class A compounds), 90 Shore A grades (NBR B1001, B1006) handle higher extrusion pressure, and specialty 50 Shore A NBR is used when closure force must stay low [S4].
Specification and sourcing signals to track
For procurement, the verifiable signals to watch are compound certifications rather than trade names: FDA 21 CFR 177.2600 NBR compounds (B1002, B1006, B1105) for food contact; NSF 61 (B1058) and NSF 51 (B1122) for potable water and food equipment; AMS-P-25732 and MIL-P-83461 military NBR grades for aerospace hydraulic service; and UL 157 listings where flammability-rated seals are required [S4]. On the FKM side, monitor whether the application needs the broader temperature envelope of FKM versus HNBR (improved hot-oil NBR for 135°C) versus FFKM (continuous chemical and thermal service to about 340°C at very high cost) before paying the fluorocarbon price premium [S3][S4].