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It is absolutely imperative that you refill the gear oil in your car's rear differential with the correct type of fluid. Refer to the owner's or service manual for your car in order to find the correct type of gear oil to purchase. You can also ask the clerk at your local auto parts store to assist you in finding the right gear oil. Purchase gear oil that comes in a squeezable plastic bottle with a spout unless you have an oil pump to use. Open the container of rear differential gear oil and insert the end of the long spout into the open filler/service port. Squeeze the bottle slowly to push the oil out of the bottle and into the differential until fluid begins to leak out of the hole. Make sure to wipe up any fluid that leaks out of the open service port. If the differential takes an entire bottle of gear oil, you may want to have the differential looked at by a mechanic to address any possible leaks. Once the differential is full, reinsert the plug for the filler/service port and turn it clockwise (to the right) until it is tight. Use the appropriate wrench to tighten it enough to ensure it won't vibrate loose as you drive. Do not overtighten the plug, it needs only to be tight enough to be secure. Remove the jack stands and lower the vehicle back to the ground.
Purchase the correct type of gear oil. Insert the spout into the filler/service port. Replace the filler plug.