an improved oil tank for storing hydraulic fluid and the hydraulically - powered system is fabricated from a polymeric material which is resistant to chemical breakdown from oil . the tank is constructed in a manner which facilitates internal circulation of hydraulic oil during use . the novel configuration of the tank lends itself to inexpensive fabrication through a rotational molding process . the tank is lightweight , inexpensive to manufacture , and is particularly advantageous for use in vehicle - mounted hydraulic drive systems .