I've been restoring a 77 pontiac CAN AM over many years very slow hobby of mine. I fired it up a few times since I'm coming to a close on it and it sounds great. I decided to purchase a new three core AL RAD. And it came today. I took off the old rad. Cap and saw it was all mucked up in side the color was not dark brown but kind of rust mud light brown if that makes any sense. I am worried its oil. I checked the trans fluid it looked light clear red. I took off the thermostat and look at the fluid in the intake it was crystal clear as the day I put it in as if nothing from the old rad was mixing or getting passed the thermostat. I don't want to put the new one in if it's going to end up like the old one.Any help would be appreciated.
Bart

Does the radiator you are removing have a built-in oil and/or transmission cooler?
You can tell if there are little lines or pipes running into the end tanks on the radiator.
If you have those, one may be seeping either oil or trans fluid into the coolant.
Whether the potential leak is oil or trans fluid, both operate at a higher pressure than the cooling system. It also looks as though you may have caught the fault in the early stages.