While a very little bit of oil mist getting into the airbox is normal, not nearly the amount you have. The airbox has two drain hoses. One is a water drain always open from the "cup" in the middle of the air filter. The second is an oil drain that run from the low point beneath the carb inlets down to near the kickstand and should have a rubber plug for periodic draining. Now back to your problem...there are two ways that large a quantity of oil can find itself into the airbox: The engine oil was grossly over filled. This is actually rather common since many people do not know how to properly check and change the oil level on this dry sump system. The bike must be run, preferably until fully up to temp, before checking the oil. Then unscrew the oil level dipstick, wipe it clean and check level without screwing dipstick in. Similarly with oil changes, BOTH engine and frame tank have been drained before adding 1.6L to frame oil tank. Then run the engine for a 2-3 minutes before adding the remaining 0.6L of oil. Getting the oil level wrong and the excess oil blows out of the engine oil breather into the airbox. Oil breather and feed hoses have been swapped (possible after an engine removal to do a head gasket). The engine has two oil breather hoses. A small breather hose should run between the valve cover to the frame oil tank. The second and primary breather should run from the front of the right crank case cover (just ahead of stator and below the "Aprilia" logo by the starter) to the left side of airbox connecting just ahead of the battery. This primary breather can be easily mistaken and swapped with the oil return hose (bottom left side of engine case to frame oil tank). Getting these hoses mixed up will result in the scavenge oil pump filling the airbox with oil and eventually starving the engine of oil (oil pressure light would illuminate first). Have you done an oil change yet? I always start off with a used bike by checking all the fluids because I do not trust anyone else's abilities in maintaining a bike!