1996 e300 transmission
 

Transmission has synthetic in it and by accident i put a half of a quart of regular atf in it is my tranny going to die

Yours takes Dex 3.

Its a 722.438, not a 722.608 (which also uses D3 BTW, BUT with special MB additives not available elsewhere).

thank you so much you made my day

Is the regular atf mineral based?

Won't mixing mineral and synthetic create similar issues to those faced when doing the same in an engine?

AFAIK there is no negative consequence to mixing synthetic and conventional oils or tranny fluids. In fact, there are blends on syn/con oils on the market and if you look at the bottles they usually claim the syns are compatible with conventional ones. They would almost have to be as it is impossible to get all of the fluid out of a transmission and if they wanted you to switch to Mobil 1 how could they market a product that was incompatible with the residual conventional fluid in the engine or transmission?

I assume the original poster used a Mobil 1 or equivalent synthetic version of Dexron III? If so I would not worry about mixing any conventional dexron III with it. See the data sheet:

http://www.mobil.com//USA-English/Lubes/PDS/NAUSENPVLMOMobil_1_Synthetic_ATF.asp

Bottom entry under "features": "Compatible with mineral ATF fluids and all common seal materials "