I am in the process of repairing my 1997 Toyota 4-runner with a v-6 engine. I have only had the vehicle for a couple of days. I was doing a routine maintenance check on all the fluids. What I found was the antifreeze to be white in color with a brown oily substance on the reservoir. The rest of the fluids in the vehicle where all clean. Oil was very clean and no signs of white or discoloration, smelled normal. Transition fluid was clean, had a normal reddish color, did not smell burnt or bad. I performed a radiator fluid to clean all the white and slug out. I refilled with all purpose 50/50 antifreeze. After driving for about a half hour I recheck the antifreeze and the reservoir had a brown oily residue on the sides. I opened the cap and ran the engine without the cap. The antifreeze that spilled out of the radiator had this brown flaky oily substance in it. All other fluids where rechecked and still clean. I have reason to believe that it is the radiator leaking or the oil cooler leaking. I am trying to figure out if it is transmission fluid or engine oil in the antifreeze? that should tell me if it's the radiator or the cooler? or are they the same thing. Is the cooler located in the radiator?
I think the likely suspect here is engine oil. If your head gasket is blown it is not unusual for oil to get into the coolant or the other way around. Coolant is being circulated through the engine very close to where oil is being circulated through the engine. Consider doing a compression test and/or a leak down test.
Flush the radiator. Some of the red coolants, especially if they used dexcool will look like that normally. If you've got oil, or transmission fluid going into the coolant you will likely have coolant going into the oil or transmission fluid, so check those fluids closely for contamination. Once you've done that, monitor the coolant closely and see if the condition comes back.
On a side not if you bought this vehicle recently from anyone but a private seller, or auction house take it back to them for analysis before doing anything else. They make take care of it for you.