Choosing the appropriate viscosity grade of fluorosilicone oil for sealing and lubrication is crucial, balancing fluidity (low viscosity) with oil film strength (high viscosity). Fluorosilicone oil combines the high and low-temperature resistance of silicone oil with the oil and solvent resistance of fluorinated compounds.
The following is a selection guide based on application scenarios, temperature, and operating conditions:
1. Core Selection Principle: Application Scenario
This is the primary factor determining viscosity, as sealing and lubrication have different requirements for viscosity:
A. For "Sealing" (e.g., O-rings, gaskets, rubber parts)
Recommended viscosity: Low to medium viscosity (50 to 500 cSt @ 25℃).
Reason: Seals usually require lubricants with good penetration and compatibility. Low-viscosity fluorosilicone oil can better wet the rubber surface, preventing drying and aging of the rubber, while not causing excessive assembly resistance.
Typical choice: 100 cSt or 350 cSt fluorosilicone grease base oil.
B. For "Lubrication" (e.g., bearings, gears, slides)
Recommended viscosity: Medium to high viscosity (500 to 1000+ cSt @ 25℃).
Reason: Lubrication requires the formation of a stable oil film to carry the load. If the viscosity is too low, the oil film is easily broken, leading to wear; if the viscosity is too high, it will lead to excessive starting torque and increased energy consumption.
Typical choice: 500 cSt (general purpose) to 1000 cSt (heavy load or high temperature).

2. Key Parameter: Operating Temperature
The operating temperature range of fluorosilicone oil is typically -60℃ to +200℃ or even higher (some can reach 250℃). The viscosity selection needs to cover both ends of this range:
Low-temperature starting performance (-40℃ to -60℃):
At extremely low temperatures, all oils become thicker. If the equipment starts in a cold environment, a lower viscosity grade of fluorosilicone oil must be selected; otherwise, it will lead to difficult starting or insufficient lubrication. Recommendations: In low-temperature environments, prioritize products with base oil viscosity between 100-350 cSt.
High-temperature stability (>150℃):
As the temperature increases, the viscosity of the oil decreases. If working at high temperatures, a higher viscosity grade of fluorosilicone oil should be selected to ensure that the oil film still has sufficient thickness to protect the components at high temperatures.
Recommendation: For long-term operation at around 200℃, it is recommended to choose fluorosilicone oil with a viscosity of 500 cSt or higher.

3. Operating Conditions and Environmental Factors
Contact with Chemical Media (Fuel, Solvents):
If the sealing or lubrication area comes into contact with gasoline, aviation kerosene, or strong solvents, ordinary silicone grease will swell and fail, while fluorosilicone oil is the preferred choice.
Selection Tip: In environments where contact with solvents occurs, the viscosity should not be too low (recommended >350 cSt) to prevent rapid loss of grease after solvent dilution.
Load Size:
Light load/precision instruments: Low viscosity (50-100 cSt) to reduce resistance.
Heavy load/impact load: High viscosity (1000 cSt+) or directly use fluorosilicone grease with added PTFE (polytetrafluoroethylene) solid lubricant; fluorosilicone oil alone may not be able to withstand extremely high loads.
Volatility Requirements:
High-viscosity fluorosilicone oils (such as 1000 cSt) usually have lower volatility and a longer lifespan under high-temperature, long-term operation.

4. Reference for Common Fluorosilicone Oil Viscosity Specifications
The common fluorosilicone oil specifications (kinematic viscosity at 25℃) on the market are as follows. You can "match" your needs based on the following standards:
Table
Viscosity Grade (cSt @25℃) Typical Application Scenarios Recommended Use
50 - 100 Low-temperature sealing, defoaming agent, diffusion pump oil Lubrication of rubber parts in low-temperature environments, occasions requiring extremely low torque.
300 - 500 General sealing grease base oil, solvent-resistant lubrication Most commonly used. Balances fluidity and oil film strength, suitable for most O-rings and medium-speed bearings. 1000 and above: High-temperature lubricating grease base oil, damping oil.  Suitable for high-temperature valves, heavy-duty low-speed components, and applications requiring long service life and no leakage.

5. Summary and Recommendations
If you are unsure which to choose: Prioritize trying 350 cSt or 500 cSt fluorosilicone oil, which are the most widely used intermediate values in industrial applications and have the strongest adaptability.
For seals, prioritize low viscosity: To protect rubber and facilitate assembly, seals usually require lower viscosity than what is needed for lubrication.
Note the distinction between "oil" and "grease": If you need the lubricant to remain on the component for a long time without leakage (such as in bearings), it is recommended to purchase fluorosilicone grease, which is made by thickening fluorosilicone oil with a thickener (such as PTFE or silica), rather than pure fluorosilicone oil, as oil is prone to being thrown off or leaking.