"I'm not sure what brand of tranny fluid they used. Could that make a significant difference?"
There was a big discussion recently on this. But basically, Mercon is not acceptable anymore.
"Idle speed at startup is about 2500 RPM (cold)"
It would be hard to get in 1st or Reverse with such a high RPM.
"I have been having codes for the O2 sensor again, causing the rpm to drop too quickly btwn gears and sometimes stall at idle. But that I will soon fix myself, and assume that is unrelated to this issue."
This needs to be taken care of. It could be related.
"Should I have the tranny fluid drained and refilled with a proper brand?"
Can't hurt.
