In Reply to: Re: Hydraulic Fluid Sources and Type posted by Kimberly on March 11, 2019 at 12:13:17:
Both of those are fine and I admire people that want to learn new things. Your confusion is natural because unless you have been studying hard for your tribology exam the SAE viscosity grades are not intuitive. People tend to think they are viscosity measurements and they are not - they are simply grade names that happen to be numbers. They could just as easily be labeled light, light-medium, medium, medium-heavy, and heavy. For many people that would be a lot less confusing. Plus the grade numbers are different for engine and gear oils. For example the specified viscosity of SAE 40 engine oil is 12.5 - 16.2 cSt @ 100C which almost completely overlaps the viscosity range of SAE 90 gear oil which is 13.5 - 18.4 cSt @ 100C!!!! On the other hand the ISO viscosity grades are actual viscosity measurements. An ISO 32 hydraulic oil has a measured viscosity of 32 cSt @ 40C with an allowance of +/- 10% (29.8 - 35.2) Notice that viscosity is measured at a different temperature than the gear and engine oils so there is no direct comparison between the ISO and SAE systems.
If you are like me and don't have a life and spend hours reading datasheets and technical specifications you will discover that the Premium UTF's generally have a measured viscosity of ~9.5 cSt @100C and ~55 cSt @ 40C. That puts them somewhere between an ISO 46/68 hydraulic oil, on the low end of an SAE 30 engine oil, and smack in the middle of an SAE 80 gear oil. That is one of the reasons they are excellent oils for use in common sump tractors.
Now I bet you are even more confused ;-)