Im not sure about that model, but transmission coolers are normal in a radiator , I dought it is optional as its internal and should not be disconected unless you live in the artic , heat kills transmissions fast and the radiator should be replaced. also if you live in a hot area an additional cooler can save your trans from early failure. If there was oil in the water than there is probably water in the trans also . Everything , motor and trans should be flushed. And If im right a new mechanic found. My 91 has the cooler standard in the radiator. and I plan in adding an aditional unit as I have done on other cars. The problem is cars dont come with trans temp guages, and if you drive hard or tow , they realy are a necessity to avoid early failure. Ive screwed
2 toyota transmissions already from heat.