Use a gasket or 100% silicone to seal the cover or third member to the housing. Use only high quality name brand gear oil and fill the differential until the oil spills out the plug hole. Positraction carriers require anti-friction additive.
All new gear sets require a break-in period to prevent damage from overheating. After driving the first 15 or 20 miles you must let the differential cool for 30-40 minutes before proceeding. Drive 500 easy miles before towing. Tow for very short distances (less than 15 miles) and let the differential cool before continuing during the first 45 towing miles. Change the gear oil after the first 500 miles. This will remove any metal particles or phosphorus coating that has come from the new gear set.
ANY OVERLOADING OR OVERHEATING CAN CAUSE THE GEAR OIL TO BREAKDOWN AND THE RING & PINION CAN FAIL.