How to define a new solvent in Gaussian09 to run SCRF?
The correct input should be more like (replacing your Water2 with Generic)

#p b3lyp/6-31G(2df,p) int(grid=ultrafine) SCRF(Solvent=Generic,Read)

Comment

0 1
...

stoichiometry=H2O1
solventname=Water2
eps=78.3553
epsinf=1.77556


as it is stated in the Documentation under Additional Input for PCM Calculations.



Note - this only gets your calculation running. Your shown current input section might be insufficient to properly define sulphuric acid.

As Geoff noted, the Gaussian Documentary states an important fact short before showing some $\varepsilon$-values of common solvents that should be taken into account.


  We list the ε values here for convenience, but be aware it is only one of many internal parameters used to define solvents. Thus, simply changing the ε value will not define a new solvent properly.