I just got myself a second hand car with about 100K mileage, which I presume that the previous owner never change rear differential fluid. Is it safe to change? If I change it now, can it cause any damage or slippery like changing a very old transmission fluid?
No. You can change axle oil any time.
cause any damage or slippery like changing a very old transmission fluid?
changing transmission fluid can not damage a transmission. If it slips then the transmission is already worn out, as is explained in our FAQ.
Is it safe to change?
Yes, differential is splash lubricated, there's no pump, etc, and any contamination usually falls to a magnetic drain plug. All oils wear out, through thousands of hot/cold cycles.
If I change it now, can it cause any damage or slippery like changing a very old transmission fluid?
That is only with neglecting automatic transmission fluid and then changing it. There can be grit that was originally on the transmission's clutches if neglected. If you change transmission fluid after the clutch materials have worn off, they will slip. Manual transmissions use a mechanical clutch, and they're splash lubricated as well.
