2004 Pontiac Grand Prix - Dirty coolant

Hi there. My car’s coolant gets dirty with what seems to be an oily substance, even if I do a coolant exchange it gets dirty again. The funny part is that both my engine & trans dipsticks show no signs of coolant whatsoever, neither in color nor in smell.
The only leak I’ve spotted though is from the power steering assembly. Any ideas on what might be causing this problem? Thank you very much!

There may be a very small leak in the transmission fluid cooler inside the radiator.

The transmission fluid pressure can get high at certain RPM’s


Where as, the cooling system pressure is lower than what the pressure cap reads.

So, I think the tranny cooler in the radiator has a leak.