The recommendations I have seen is 1 liter/min for a minute in 5 gallons of regular strength beer. I use that for my lagers. For 2.5 gallons cut the time, or the flow, in half.
I was using O2 for everything, and found my British ales that should have ester character to be too clean. Now those just get air from pumping over to the fermenter, no shaking or mix stir.
Edit - if racking from the kettle to the fermenter, then I would use the mix stir or shake.