Front diff oil saved during axle seal replacement?

Approximately a month ago my driver side axle seal went and it was replaced under warranty. Yesterday I went for an oil change (engine oil) at a different shop and the guy showed me a piece of paper with 3 dabs of oil on it. He said they were from the front diff, rear diff and transfer case. Two were golden brown and the other was almost black. He said the black one was the front diff and suggested I get the gear oil changed.

Is it possible that the gear oil was saved during the axle seal replacement and now needs to be changed or was that guy trying to up-sell me to something I didn't need by putting a dab of used motor oil on a piece of paper?

 

I would bet they never changed the oil. They just popped a new seal in and topped it off