I've had an 03 P 71 Street Appearance Crown Vic for 4 years. Had about 93K when I got it; now has 125K. Car was a detective's car, not beat up or abused. The agency that had the car deletes the computer records once it has been sold, but I've talked with the fleet manager soon after I got the car, and he says they go through their cars at 60K (tune up, etc.), due to a hot climate and extensive idling.
Now, I've heard/ read all kinds of advice on adding friction modifier to the differential. I think the car came with 75W140? gear lube.My car is having no symptoms of trouble. No noise or noticeable slippage, etc. I can do low speed lock to lock right and left "donuts" with no noise and the rear is smooth while doing this. But I read that I should add 4.5 Oz FM at 100K ( it's past that, I know); also read that FM is not needed if there is no chatter or other noise in the RE.
So, which is it? Add or not? Wait for symptoms then add?. I use snow tires in winter; wonder if that changes things? Thanks.

 

The only time that you would need to add additional friction modifier is if your start to hear chattering from the clutch packs in the rear-end when making normal turns. In the case of my 88 Ranger, the rear-end would actually groan when making turns. At this point, I would just replace the lube in the rear-end with a fresh dose of gear lube and friction modifier as the noise is telling you that lubes additive package is getting pretty much depleted and the lube is loosing it's ability to do its job.