What wire should I use for a sauna 100' away from the panel?
Having run a few of these calculations, the break point is usually a little past 100' if you want to hold voltage drop below 3%.   However, for a resisitive heater, there is no reason to hold voltage drop below 3%. It is a waste of money. 

Regardless, 8000 watts happens at 33.33 amps on US 240V* voltage.  The UL-approved product instructions call for 8 AWG wire.  Southwire's voltage drop calculator tells me those conditions at 100' run yield 1.92% voltage drop on 8 AWG.  

If the hot tub failed on 2% power drop, there is no way UL, CSA or TUV would list it.  

You're all set.  



* power was 110/220 when AC power was first mass marketed in 1915-ish, and the term stuck. Today,  US voltage is 120/240.  Except in Puerto Rico, where it's 0!