Unfortunately that sounds like transmission fluid. If you don't have dipstick that means you have an 8HP70 transmission. Here is a video and the written instruction on how to check the fluid.
If you notice in the video the color of the fluid he is using. It is a greenish color and feels very much like mineral oil. It is not traditional pink/red transmission fluid. If mixed with coolant it can darken in color.
I have seen a few of these where the cooler cracks and the trans and coolant mix. IF this is the case, it sounds like you caught it at the beginning stages.
If it were me, I would pull that plug and inspect the fluid in the trans just to be safe. Obviously if you do this after the leak down test, that would be fine because if you find a head gasket issue then this would confirm engine oil.
Let me know what you find. Thanks
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Friday, June 7th, 2019 AT 6:38 PM