A lubrication system (100) for internal combustion engines, in particular for vehicles with a rideable saddle, said system comprising a crank chamber (110) in which at least one connecting rod is accommodated, and a basin or pan (120) for collecting a lubricating liquid (LL), wherein said crank chamber (110) and said collection basin or pan (120) are put in mutual communication by means of a first opening (111) obtained in said crank chamber (110), wherein the flow of said lubricating liquid (LL) through said first opening (111) from said crank chamber (110) into said basin (120) is adjusted by means of a first adjusting element (112) adapted to be switched to an open position so as to allow the discharge of said lubricating liquid (LL) from said crank chamber (110) into said collection basin or pan (120) at a predetermined switching gas pressure value inside said crank chamber (110).