Varistor S10K30 for 24-40VDC input?
The data sheets are telling you this: -


The device will conduct 1mA somewhere between 47-10% and 47+10% at 25ºC
At 5A the voltage across the device will be no more than 93V (this is a pulse test)
The max impulse current (based on 8/20us pulse) is 500A at 85ºC
At 85ºC ambient watch out because the operating voltage level drops to 38Vdc


You also need to look at the VI curve for this device: -



The above will tell you what to expect for two conditions. The left hand area marked A is for quiescent conditions (leakage current) and at 40Vdc there may be a current of about 500uA flowing. This is OK for the design unless you are operating super low power. The right hand area is the protection side of the graph - note how the device has a staggered curve where left and right cross-over. This is giving you the 93V max at 5A figure and you can use the curve to predict what the worst case terminal voltage will be clamped at (say) for 1A. To me this looks like 82V.

My conclusion is that it is OK for a 40V supply unless you are operating at a high ambient temperature - this could give a clue about where/how you should mount the device on the PCB - If the PCB is vertical, maybe it's at a low part of the board where temperatures are bound to be lower. Maybe it's near to a fan. You need to make these decisions.