I would like to test for viscosity of automotive lubricant oil over time and would like advice on the best temperature to test it on. Is 100 deg C a better choice than 40 deg C (as operating engine temperature is around 100 deg C)?

Thank you.

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.