Viscosity test

Historically labs would normally test at 40°C only. As equipment became cheaper labs would also test at 100°C - thereby also gaining the Viscosity Index - which helps tremendously.

Assuming you have 1 instrument (and no time to continually ramp between the 2 degrees) : I would suggest that 40°C would the better option. The majority of literature, spec sheets for oils, fuel dilution to viscosity numbers etc. is written around the 40°C temp. Also better for hydraulic and gear oils.

Furthermore I find readings more accurate at 40°C.

HI John
We have an easy and rapid instrument solution for testing at 40 and 100 with VI calculation.
I can't joint here a document :SVM 3000 - customer profile example for oil service laboratories.
Simply let me know if you have further questions
Regards
Sylvain

I agree with John.
Engine oil operating temps are around 100° C and viscosity of reciprocating engine components at that temp are more critical than being measured at 40° C.

KV 100 deg for Auto, because, SAE refer to this value. But, in order to get VI, KV 40 deg C has to be tested.

Hello John Low,
your test should be designed to provide you with the answers that you are trying to understand. So what are you trying to understand? If you are running a multi-grade and you want to identify the point when the VI is starting to breakdown then you need to perform testing at both temperatures. If you are looking for fuel dilution then testing at 40 would be fine. Most labs will give you the option of what temp you wish to have reported and some even provide both and the VI as standard reporting.
hope that helps

The SAE J-300 engine oil viscosity standards show a hard spec for 100C as well as the CCS & the MRV at lower, grade-specific temperatures. J-300 doesn't address the 40C viscosity directly at all and only gives it an oblique nod by requiring a minimum VI. Most suppliers will show a hard spec at 100C and a "typical" at 40C.
So, by tracking at 40C you can observe trends and even establish some statistical tests, but you won't be able to say definitively that the oil is still within its original grade, if that matters to you.
Of course any industrial oil viscosity testing would have to be at 40C to be of real use.