transmission cooler cracked and fluid mixed into radiator coolant. replaced radiator, water pump and hoses. same thing happened with new (not oem ) radiator within 1k miles. anyone know what would cause this?

It takes a lot of time to get oil out of a cooling system. Dish washing liquid + water + drive around for a few days tends to work. Repeat as needed. ( Assuming that the new rad is OK )

97SL320 is right.

you will need to run a degreaser in the system for a while to get the oil out, drain everything and use a bottle of shout in water and run the system for an hour or so. Or use commercial cascade dishwasher soap - It also cleans out greases.

Some truck shops use simple green and water in the system after similar failure.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)