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I am a layperson, not an engineer. After reading some articles on what ISO viscosity grades mean, I do not understand what it means for an oil to be labeled with an ISO viscosity range, e.g. "ISO 32-46". I thought the ISO number translated to a specific flow rate under a defined set of circumstances relating to temperature and orifice.

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  • $\begingroup$ Tolerances are a thing. You might be concerned with just one bottle of oil, but the manufacturer needs to deal with hundreds or thousands over multiple batches. $\endgroup$
    – DKNguyen
    Jun 15 at 13:42
  • $\begingroup$ @DKNguyen : Are you saying the oil so labeled would be "close enough" to 32 and close enough to 46 (i.e. somewhere between them) that it could be used when either viscosity was called for? $\endgroup$
    – Tim
    Jun 15 at 13:50
  • $\begingroup$ Yes in that it might vary from bottle to bottle. No with regards to "close enough to use." Whether you can use it depends on how tolerant or sensitive your application is. You can probably get away more with a lawnmower than a race car. Ratings generally do not tell you how to use the product. They tell you what the product is. $\endgroup$
    – DKNguyen
    Jun 15 at 14:11

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In the US, motor oils are sold with viscosity ranges ascribed to them if they contain additives that flatten the viscosity-versus-temperature curve (so-called "multi-viscosity" oils). Is the ISO rating you are citing here describing a multi-vis product?

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  • $\begingroup$ The oil in question is pneumatic oil. The tool calls for ISO 46. When searching for a small quantity of that oil, I found products labeled ISO 32-46 and I didn't know what that range signified versus the simple designations ISO 32 and ISO 46. In practical terms, what does it mean "to flatten the viscosity-versus-temperature curve"? Does it mean that the viscosity of such treated oil remains relatively constant despite changes in temperature, and if so, where in the range does the viscosity lie? Does ISO 32-46 have a relatively constant viscosity of about 39 across a temperature gradient? $\endgroup$
    – Tim
    Jun 16 at 13:28
  • $\begingroup$ ...across a given temperature gradient? (ran out of characters) $\endgroup$
    – Tim
    Jun 16 at 13:33
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    $\begingroup$ Multi-vis oils retain their low-temperature viscosity at high temperatures (the slope of the viscosity-versus-temperature curve is decreased). so they do not lose lubricity when running hot. I do not know if the oil in question is such an oil. $\endgroup$ Jun 16 at 15:05

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