Dewaxing aids used to assist in the solvent dewaxing of waxy hydrocarbon oils are recovered from the dewaxed oil or waxy precipitate, or both, by selective permeation of the oil or wax through a perm-selective polyimide ultrafiltration membrane, under ultrafiltration conditions, leaving a retentate of dewaxing aid, of reduced oil and/or wax content, which dewaxing aid-rich retentate can be recycled to the solvent dewaxing process. The polyimide ultrafiltration membranes are preferably produced using a casting solution of polyimide polymer dissolved in a dual solvent system comprising a pro solvent/anti-solvent pair at a ratio of 10:1 to 1:1 and further containing an inorganic salt, said casting solution being used to produce a film which is permitted to evaporate for 1 to 60 seconds and then quenched in a quenching solvent.
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