if the OP brought the Z in for oil change and all the sudden the SA tells him "oh.. you need to change the sensor..."
then I'll be
how would you do a diagnosis that should cost $80 but now for free and then tell me I need to pay $800.
was your Z behaving normally before you brought in to the dealer?
I will suggest you bring it to a reputated local mechanic and get a 2nd opinion.