The invention concerns a vent cap for oil tanks of machines and motor equipment, comprising a cup (1), equipped with means of engagement (4) with the seat of the tank filler, and a lid (12) mounted on the cup and which snaps thereon. Between the cup (1) and the lid (12) there is a gap allowing the vapor outflow from the tank to the outer environment or viceversa. According to the invention, said lid (12) comprises two cylindrical diaphragms (12b, 12c), concentric to the cap axis and which, in cooperation with the cup (1), form labyrinth means for the flow of vapors and air. Cup (1) also carries a guide (7) supporting a level rod (8), consisting of a central, cylindrical main body (7b), wherefrom ribs (7a) for anchoring to the cup (1) depart. Guide (7) extends, at the top, into two flexible flaps (7c), provided with means of engagement (10) of the rod (8).