Take it to a shop. Have a transmission service and flush done. The will hook up you transmission cooler lines to a fluid exchanger so it will be able to put in new oil while at the same time take the old oil out. Your 1991 Ford Probe will take Mercon/Dexron III ATF
Check at your local Autozone store. They will tell you what fluid type you need, What capacity. And sell you a filter with gasket. Sometimes they can print out repair guides for free. Or sell you a Haynes manual that will walk you through the change.
