PURPOSE: To obtain the subject terpolymers excellent in the improvement of the flowability of mineral oils by polymerizing ethylene and a mixture of a predetermined amt. of vinyl acetate and a predetermined amt. of i-butyl vinyl ether under a specific condition in the presence of an initiator and a mol.wt. adjusting agent.

CONSTITUTION: Ethylene, 5-20 mol.% of vinyl acetate and 0.2-5 mol.% of i-butyl vinyl ether are polymerized under pressure of 50-70 MPa, pref., 150-300 MPa at 100-350°C, pref., 120-325°C in the presence of an initiator such as oxygen under the addition of a mol.wt. adjusting agent such as propionic aldehyde (pref. use amt.; 0.05-10% by wt. of monomers) to obtain an objective terpolymer with an average mol.wt. (Mn) of 500-5,000 g.mol-1. This terpolymer is added in an amt. of 0.005-0.5% by wt. of mineral oil distillates to improve flowability.