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I am guessing that they were trying to save you some money, and use a less expensive fluid for a short "flush" run.
Mercon (now Mercon V) is a FORD specification.
Meaning that FORD came up with the specification, requirements, and licensing to companies to sell fluid that states "meets Mercon Specification" on the lable.
Dexron is a General Motors, GM, specification.
While the specifications may be different, it is possible for 1 fluid to meet both Mercon and Dexron specifications.
A good start, as UnDummy mentions is the transmission dip stick and / or owner's manual.
Then you would look for a fluid that lists that on the label.
I would also STRONGLY recommend checking to see if Lexus has issued any Technical Service Bulletins, TSB's, for your vehicle.
This is particularly good to do for our older vehicles.
As new, often improved, fluids come out, manufacturers often issue TSB's stating that one SHOULD or MAY use the newer specified fluid......and easy one would be Dexron II may have been the latest specification when one's vehicle was manufactured, but then Dexron III came out.....which is an improved fluid that would be an upgrade to the Dexron II.
However, an newer fluid is NOT automatically better for an older vehicle......as in the case of ATF for the newer Constant Velocity Transmissions not being appropriate for other transmissions.
Another thing to take note of.
Some "smaller" manufacturers of ATF will state "recommended for Mercon V (or Dexron xx) applications"
This is because they have not paid the money to be a "licensed" fluid...which requires royalty fees, and .....very expensive testing processes to get.
This does not necessarily mean that they are not good fluids, just that the costs are very substantial for a smaller company.
Amsoil and Redline are 2 companies that manufacture high quality fluids, that may not be "licensed"....but are certainly not "no name" cheap knock off's.