Used R160 4.44 suretrac rear diff
 

Would a second hand (probably from P1) R160 4.44 surectrac diff, which is almost 20 years old be an improvement over stock viscous diff (with similar mileage)?
Does it wear over time, does it work weaker with increasing mileage? Or is it like quaife which is advertised as non-wearable with lifetime warranty?
Is it a servicable item? The one that I could buy is whining, but the seller says that apart from that it works great. He says that after changing some bearings it will be 100% healthy. Is he right? Do you think it is only bearings or main mechanism could be faulty and non serviceable?