93 LeSabre - brown oily film in overflow jug

There is a brownish-oily feeling film approx. 1/2" thick laying on top of the antifreeze in the coolant overflow jug on my moms 93 LeSabre. Anyone ever see this before? I have a hunch that it's tranny fluid and that there is probably a leak inside the radiator tank. Any input would be greatly appreciated!

Are you loosing trans fluid, some seperation occurs over a long period, it could be normal, or as you say it is oil. What is in the radiator?

Someone told me he can usually tell if the UIM gasket has failed previously by looking at the color of the coolant in the overflow. Not sure if that is true for a 93 though. Seems like that is quite a bit of oil.

Check all fluids for shortage.

Was the car running Hot?
What doe's the coolant reservior tank smell like?
Gas, oil, or coolant