Transmission and axle lubricants accumulate contaminants because they are subject to heat and mechanical stress. They also have additives that need to be replenished. Don’t use transmission and axle lubes interchangeably.
1. Park your truck on a hard, level surface. Chock the wheels. Place a container of at least 30-quart capacity under the rearward rear axle.
2. Put a wrench into the square hole in the drain plug and turn counter-clockwise to loosen, then remove by hand. Inspect the plug, which is magnetic, to see what kinds of metal filings have stuck to it. A thin, grainy film is okay, but large pieces of metal are a potential sign of trouble. Wipe the plug and the threads inside the drain hole with a clean rag, then carefully screw the plug in straight and snug it up.
4. Fill the differential with the recommended lube. Your dealer can provide lube type and capacity specs, based on the vehicle’s serial number. Check with your finger to make sure the liquid level is up to the bottom of the hole. Reinstall the filler plug and inspect the drain plug for leaks. Repeat the entire process on the forward drive axle
5. Empty your drain pan and place it under the transmission. Remove the drain plug. Clean the plug and look for metal filings before reinstalling. Clean around the fill plug, which is located on the side of the transmission.
6. Fill with about 22 pints of the recommended fluid, normally a fully-synthetic 50-weight transmission fluid.
7. Drive the truck a short distance. Then inspect the drive axles and the transmission again for leaks.
Mineral vs. synthetic
Don’t be fooled by the price of synthetic lubes: Per 1 million miles, mineral lubes have to be changed about eight times, versus once for synthetics. Synthetics also protect moving parts better.