To replace the motor oil of an Aprilia SR 125, start by parking the scooter on a flat and stable surface. Lift the front part using a center stand or an appropriate support to ensure that the engine is in a horizontal position. Make sure you have a container for the used oil and a suitable wrench to remove the drain plug. After preparing everything you need, locate the drain plug on the engine, which is located on the bottom of the fairing. Place the container under the plug and carefully unscrew it, allowing the old oil to drain completely. This process may take a few minutes to ensure that all the residual oil is removed. Once the draining is finished, reposition and tighten the drain plug, making sure it is well sealed to prevent leaks. Next, proceed to fill the engine with new oil through the designated filling opening. Use the oil recommended by the manufacturer to ensure the correct operation of the engine. After filling the oil, start the engine and let it run for a few minutes to ensure that the oil circulates well inside the engine. Turn off the engine and check for leaks under the scooter, making sure the oil level is appropriate. Finally, dispose of the used oil according to environmental regulations and regularly check the oil level to maintain optimal engine performance.