What kind/brand did you buy? Where did you mount it on the radiator, or was it somewhere else? I bought a B&M cooler when I had my s10, and that thing was sweet. Even when offroading in summer at rausch for hours, the trans never gave me problems. Even with my heavy foot. I think it was close to this size.

B&M 70268 - B&M SuperCooler Oil Coolers - Overview - SummitRacing.com