Fluid Like Based Period OK To Mix Synthetic And Petrol-based Transmission Fluids?

OK to mix synthetic and petrol-based transmission fluids? - fluid like based period

I have a '97 Ford Contour GL, manual transmission (MTX-75), 101K.
I want to drain the fluid and filled with Mobil 1 or Redline synthetic ATF, but I wonder if it OK to have a mixture of synthetic gasoline + liquid, provided that the liquid fuel, it is based.

I say this because it is very difficult to 100% of the liquid, can be reached by using the drain screw.