Doing a radiator flush on my Bronco. Got it all nice and clean. Dumped some anti freeze back in. Started it up. Grabbed a half gallon I had sitting on the shelf. Took the cap off, quickly turned the jug over in the funnel and walked away. Came back , WTF is that smell?? Realized the half gallon of antifreeze was 90w gear oil I had dumped in it.
About 20 gallons of Purple Power later, it was sorta cleaned out.
Needless to say never had rust in my cooling system ever again...
.