Changing the oil in a 12.5 riding mower is a straightforward project. The 12.5 mower does not have an oil filter, so changing the oil is simply a matter of draining the old and replacing with new oil. Most manufacturers suggest using SAE 30 motor oil under normal mowing conditions. Check with your owners manual for exact specifications for your model, and contact an auto parts store for advice on where to take your used motor oil. Many auto parts will recycle the oil for you.
Park the riding lawn mower on a level part of the yard or on the driveway. Set the brake on your mower and pull the ignition key out of the ignition. Go to the hood latch and pull the hood open to the service position. Pull the dipstick out of the oil fill neck.
Locate the oil drain plug. The plug is usually on the right side of the engine between the driver area and the front tire. It will most often have a cap over a nipple fitting or may simply be a hex-ead cap screw on a round tube pointing away from the mower.
Place a container to catch the old oil near the drain tube. If you have a plug with a hex-head cap, connect a piece of metal angled under the plug to form a track that will direct the oil to the container. If you have a nipple, place a small-diameter hose over the nipple and place the other end of the hose into the container.
Turn the hex cap screw counter-clockwise with a socket wrench and remove the hex cap. If you have the nipple fitting, push the drain valve inward and turn the valve counter-clockwise to drain the oil.
Turn the drain valve clockwise and the valve will pop into the closed position. Remove the hose from the nipple and place the cap back over the nipple. If you have the hex cap, thread the cap back into the drain plug and tighten with the socket wrench.
Slowly pour 48 ounces of SAE 30 motor oil into the oil fill neck. Wipe the dipstick clean with a cloth and insert the dipstick into the fill neck. Pull the dipstick out and check the oil level. Add oil as necessary. Place the dipstick inside the oil fill neck and close the hood of the mower.