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Dextron III is not synthethic.
Dextron is a rating class that GM uses. I believe Fud uses what they call Mercron.
Mixing of different stuff in generally is not a problem, but if your doing a fluid change, you will want to use all the same stuff. Mixing can be risky.
I know what you are saying, but problem is that I don't know
what fluid is in now, and I would like to change it.
From what I've read on forum so far it's hard to change all of fluid
because it stays in converter.
There are several different types of fluid. as already been stated, you will used a Dexron III fluid, which can come in synthetic blend through Royal Purple, Redline, Amsoil, etc. Other manf. such as Chyrshler use ATF + 3, Mercon V , or Type F.
Most likely you have some knockoff brand of Dexron III. Just change the fluid with whatever you wish, as long as its Dexron III. Synthetic wouldnt be a bad idea and i think it will mix with any other fluid.
Just a note, all ATF's are compatable; however, its not recommended and should only be used in an emergency. Mixing F-type and Dextron can turn your tranny into a neck snapper or slush box. Stick with what your owner's manual says.