How much oil?A puddle,fine mist or completely soaked?
Some oil in the airbox would be perfectly normal,.....bearing in mind the many good thrashings the bikes had.
Its just a vapour that comes up through the crankcase breather.The breather vents into the airbox and then the vapour coats the sides,...and sometimes,forms puddles in the airbox.
Obviously exessive oil wont be good,....it could point to a blown seal somewhere,worn rings,......or simply that the bikes owner has'nt been inside the airbox and cleaned it out for a long time.