will mixing synthetic and non synthetic atf harm your tranny?

 

No it will not harm your tranny (assuming you put in the proper grade/weight/viscosity). They even make synthetic blend fluids that are only part synthetic.

 

Most of the makers of synthetic ATF claim that their fulid is compatible with non-synthetics. So I wouldn't worry if I just had to add some of one type to another. I would not, however, intentionally try to brew my own fluid by mixing them. You just don't know what the differences are in the additive packages. I have the same view of motor oils.
If your question is related to changing types of fluid, I would have a flush done to get all the fluid changed at once.

 

Thanks for your info. Have a good day!