Short answer
Pick the least exotic seal compound whose rated temperature range brackets BOTH your coldest start and your hottest running temperature. Buna-N (nitrile) covers -40 to 165°F and is the standard, lowest-cost choice for petroleum fluids. Polyurethane runs -40 to 200°F with far better wear resistance for heavy cycling. FKM (Viton) handles -20 to 400°F for high heat and aggressive fluids. Fluid type can override temperature — a fire-resistant or synthetic fluid may rule out a compound the temperature alone would allow.
How do you choose a hydraulic cylinder seal by temperature?
Match the seal compound to the full temperature span the fluid sees, from the coldest cold start to the hottest running condition, not just the peak. A compound qualifies only when its rated low is at or below your coldest temperature AND its rated high is at or above your hottest. Both ends have to fit — a seal that is fine hot but stiffens on a cold morning start will leak.
The three standard compounds and their rated ranges are Buna-N (nitrile) at -40°F to 165°F, polyurethane at -40°F to 200°F, and FKM (Viton) at -20°F to 400°F. These are the exact ranges our configurator and seal-temperature calculator check a build against, so guidance here and the tool never disagree.
When should you use Buna-N, polyurethane, or FKM (Viton)?
Buna-N (nitrile) is the default for petroleum-based hydraulic fluid in ordinary service — good all-around sealing at the lowest cost, rated to 165°F. Reach for it first and only step up when the range or fluid demands it.
Polyurethane adds outstanding wear and extrusion resistance and a slightly higher 200°F ceiling, which makes it the go-to for heavy-cycling rod seals even though its heat limit is only a little above nitrile. FKM (Viton) reaches 400°F and shrugs off aggressive fluids that would attack nitrile, at a higher price and a warmer -20°F cold limit — a deliberate upgrade for real heat or harsh chemistry, not a default.
How does fluid type change the seal choice?
Temperature narrows the field; fluid chemistry can eliminate a compound outright. Standard petroleum hydraulic oil is friendly to all three compounds. Some phosphate-ester fire-resistant fluids and certain synthetics attack nitrile and polyurethane, so they push you toward FKM even when the temperature would have allowed nitrile.
Water-glycol and water-based fluids, biodegradable ester fluids, and specialty chemistries each have their own compatibility rules. The safe move is to state the actual fluid when you request a build so the seal is checked against both temperature and chemistry, not temperature alone.
What else besides temperature affects seal life?
Duty cycle and side load finish the selection. A high-cycle application leans toward polyurethane for wear life; heavy side load from misalignment may call for a specific seal and bearing arrangement regardless of compound. Rod speed matters too — very fast strokes can outrun a seal's ability to shed heat, which is a separate limit from the fluid temperature.
Over-speccing a seal the application never stresses wins nothing. Pick the most standard compound that covers the range, then let engineering confirm it against the rest of the build.