To change the engine oil of an Aprilia RS 660, start by lifting the bike with a center stand to ensure stability. Make sure the engine is cold before proceeding to avoid burns. Prepare the necessary tools: a wrench for the drain plug, a container to collect the used oil, a funnel, a new filter, and the engine oil recommended in the manual. Position the container under the oil drain plug, which is located at the bottom of the engine. Use the wrench to unscrew the plug and allow the oil to drain completely. While the oil is draining, take the opportunity to check the oil filter and replace it if necessary. When removing the filter, make sure to put a little new oil on the filter seal before installing it. Once the used oil has completely drained, carefully screw the drain plug back in. Using a funnel, slowly pour the new oil through the designated fill hole, being careful not to overfill. After pouring in the oil, check the level with the dipstick, adding oil if necessary until it reaches the correct level. Start the engine for a few minutes, letting it run at idle, then turn it off and check the oil level again. If needed, add more oil to bring it to the optimal level. Finally, dispose of the used oil properly and check for any oil leaks around the plugs or filter.