Having viscosity reading @ 100 C would be your best option however, what I can say from my experience, is that visco reading @ 40 C can also be very reliable most of the time
It however depends on many factors specific to your reality and application. For instance, do you use a multigrade oil ? At some point VI improver molecules can degrade and this will affect ratio between visco at 40C and 100C.
By followinfg both, I know that in my application I basically never see VI improver degradation so 40 C visco alone is reliable. In might not be the same for you. It also depends on how critical is this reading for you. Do you need to monitor oil's multigrade capability ?
It could be worth following viscosity at both temp. for sometime and see how it correlates with each other. You'll then know if you can work with it or not.