Our coolant was contaminated with a brown sludge on top, when we had a head gasket problem. Because it was oil, it floated on top in the resevoir, and coated everything. If yours is light and in suspension, it sounds more like rust.

Oil contamination also caused 'weeping' coolant hoses. If yours are dry and look in good shape after a hot drive, it is probably not oil, or has not been a problem for long, anyway.

Steve