Engine oil viscosity: converting cSt to the older Saybolt (SUS) scale
I found a decades-old industrial lubrication chart rated in Saybolt seconds while restoring an old tractor, and every modern oil bottle only lists centistokes — the two scales don't share an obvious relationship at a gla
We write the way we argue on the bench: exact factor, one sharp trap, one worked example you can re-check.
I found a decades-old industrial lubrication chart rated in Saybolt seconds while restoring an old tractor, and every modern oil bottle only lists centistokes — the two scales don't share an obvious relationship at a glance.
The factor below is the one that ends the argument — write it once, then stop improvising mid-job.
The factor that settles arguments
This is the relationship our converters use. Screenshot it if you have to — just stop re-deriving it from a half-remembered blog post.
SUS ≈ cSt × 4.632 (approximation valid roughly above 75 cSt)
Write that line once at the top of your notes. Every later arithmetic step should point back to it instead of inventing a friendlier number mid-stream.
The mistake that keeps showing up
Applying the cSt-to-SUS approximation formula uniformly across all viscosity ranges — the real relationship bends at lower viscosities, so the simple multiply only holds reasonably above about 75 cSt.
I still write the unit next to every intermediate value. It looks pedantic until it catches the one swap that would have shipped wrong.
- Write down the known value in cSt.
- Confirm the target unit is really SUS (Saybolt, approx.), not a similar-looking one.
- Apply the factor: SUS ≈ cSt × 4.632 (approximation valid roughly above 75 cSt).
- Round only at the end, to the precision your tool or spec supports.
Worked example
An old equipment manual specifies a 65 cSt lubricant in Saybolt seconds, and you only have a modern cSt-rated oil.
65 cSt × 4.632 = 301.08 SUS (Saybolt, approx.)
That's 301.08 SUS (Saybolt, approx.) — round only to the precision your tool or spec actually needs.
I usually convert once, write the answer with units, then convert back. That two-step habit has saved more weekends than any fancy app.
Quick reference table
| cSt | Calculation | SUS (Saybolt, approx.) |
|---|---|---|
| 1 | 1 × 4.632 | 4.632 |
| 2 | 2 × 4.632 | 9.264 |
| 5 | 5 × 4.632 | 23.16 |
| 10 | 10 × 4.632 | 46.32 |
Use the table for speed once you trust the factor. Do not use it as a substitute for understanding which definition of the unit you are on.
Why the exact number matters
Using the wrong viscosity oil in older hydraulic or gear systems can starve bearings of proper film thickness or increase drag and heat beyond design tolerances.
Precision is not about showing off decimals. It is about making sure the shopping list, the machine setting, and the acceptance check are all describing the same physical quantity.
Pocket recap
Multiply cSt by about 4.63 to estimate SUS for mid-to-high viscosities, but check a proper conversion table for anything below 75 cSt.
Keep the converter link nearby for the next time the same debate shows up in a group chat. Re-typing the factor from memory is how the wrong cousin of a unit sneaks back in.
Try the converters mentioned in this article
FAQ
What is the exact cSt to SUS (Saybolt, approx.) factor here?+
SUS ≈ cSt × 4.632 (approximation valid roughly above 75 cSt)
Should I round partway through the calculation?+
No — carry full precision through the multiplication and round only the final answer, to the precision your tool, gauge, or spec actually supports.
What's the single biggest mistake people make with this conversion?+
Applying the cSt-to-SUS approximation formula uniformly across all viscosity ranges — the real relationship bends at lower viscosities, so the simple multiply only holds reasonably above about 75 cSt.
This article was written by The Turbo Unit Converter engineering desk (Applied units & field metrology) and last reviewed on 2026-07-17 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.