I spotted this oil temperature gauge for the CB400F on the Yamiya site, if my guess is correct it fits where the dipstick otherwise would sit. Has anyone got one, is it worth investing in? https://www.yamiya750.com/index.php?main_page=product_info&cPath=179_281&products_id=4119
Interesting to see what oil temperature is running at, but with no effect if you're not going to take any actions to change it.
Engine should run at 90C generally, but if too low temperature, winter for example being lower ambient (no thermostat on air cooled bikes) then a thinner oil viscosity could be appropriate, yes even with multigrade. Honda data says ambient below 15C then 20 viscosity is suitable.
If for example you, due to ambient and circumstances reliably saw 100C plus oil temps, then additional cooling and/or raised viscosity may be appropriate.
The 40 viscosity in "general" use oil recommended does actually cover that 100C scenario for high ambient temperature running.
A temperature logging ability would be of more use in tracking and diagnostic analysis than a gauge.