You don't need a digital, or even a hole in the dash. I got curious about ZF temps a few years ago, so since I already had an oil temp gauge replacing the ashtray on the console, I picked up a second sender and a rotary selector switch that in where the cigarettte lighter mounted. The sender mounts in the case close to the ring gear for max temperature sensing. On a 450-mile trip to 'Vegas, it took several hours of highway driving (speeds between 75 and 100 mph) for the ZF to come up to 170 degrees, peaking at around 175F just as we arrived. Interestingly, it took several more hours in covered parking for the temp to drop back down to ambient! I'd say don't worry unless you're running well above 140mph for an hour or so- as in an ORR event.