Yes, it's normal and vital to have oil in air box. When engine is near to overheating, oil is starting to evaporate and in form of mist is going through cranksace breather to air box where it's being mixed with air and sucked into the engine where it cools down the piston and cylinder. I was reading 'bout it and noticed on cold weather my oil level is where I left it and on hot days and stop and go city driving up to 2-3 mm down on 50 km ride.