A lot of variables here.
How hard you drive, 4S or 6S, and what kind of performance you want are important considerations.
The higher viscosity diff fluids noted above, generally create more of a 'locking' action in the front and center diff. This is done primarily to avoid excessive power bleed to the front and which helps to avoid excessive ballooning on the front wheels under hard acceleration. By doing so helps to 'virtually' affect torque split between the front and rear and allows the rear wheels to spin freely under WOT, while keeping the front rubber somewhat tame.
As for frequency...I would say every 20-25 full battery packs unless you are beasting every run, or are just fanatical about keeping your fluid fresh and clean...in which case a little more frequently.
Others may know more regarding frequency than I, but from what I have understood based on optimal tuning methodology, these numbers should ensure your diffs don't get gunked up.