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Technical Reference

4. Fluid Viscosity
Measured in centistokes (cSt) or Saybolt Seconds Universal (SSU or SUS), fluid viscosity is the resistance
of a fluid to flow. As fluid viscosity increases, the cSt rating increases. Higher fluid viscosities also mean
higher pressure drop because the thicker oil has a tougher time passing through the layer of media fibers.
Cold start fluid is a good example of highly viscous fluid. See chart below.

Filter media, amount of contamination, the flow rate, and fluid viscosity are all factors in the importance of
sizing the filter for the system requirements. Filters that are too small won’t be able to handle the system
flow rate and will create excessive pressure drop from the start. The results could be filter operation in
the bypass mode, filter failure, component malfunction, or catastrophic system failures. Filters that are
too large for the system can be too costly. Oversized filters require more system fluid and higher cost
replacement elements. Optimal sizing is best.

Viscosity/Temperature Chart

A.S.T.M. Standard Viscosity-Temperature Chart for Liquid Petroleum Products (D 341-43) Saybolt Universal Viscosity

SAE 140 GEAR OIL

SAE 20
SAE 30
SAE 40
TYPE A SAE 50
10W-30
AUTOMATIC
TRANSMISSION FLUID SAE 10 ENGINE LIQUID FILTRATION - TECHNICAL REFERENCE

MIL-H-5606
KEROSENE

DIESEL
FUEL

JP4 AVERAGE

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