A syringe containing a high water content product and for use in a drug infusion system is provided. The syringe includes a plunger carrying front and rear o-rings made of a diene rubber compound such as chlorobutyl rubber or bromobutyl rubber to impart very low gas permeability characteristics to the o-rings. The plunger may be molded as a single part or in two parts. If molded as one part with radially-engaging mold portions, the seal glands in the plunger may include parting lines from the mold. If molded as two parts with axially-engaging mold portions, the front seal gland may include a sealing surface and under cut that has no parting line. The o-rings may be surface treated with a lubricant to improve sealing where the molding process gives rise to parting lines in the seal glands.