It was probably Transmission Fluid. 80w90 has some shapeshifting characteristics when warmed up or overheated. It tends to hold that characteristic while it cools down. The reason it was dark was because it was burning or catching the grindings when you were grinding the gears.
Stalling while in neutral… not the transmission unless it has some kind of electronic feedback on the AWD. Burnt VSS (Vehicle Speed Sensor) is telling the ECU the rear wheels aren’t spinning but the front wheels are moving like a bitch, so the ECU is trying to stall out the car to match. Really not sure. I don’t know much about Nissan AWD.
Or you know, some Moron could’ve dumped Engine oil in the transmission thinking that it’s what the tranny needed. Gear oil has a horrid smell.