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