According to Raymond Murphy in his English Grammar in Use (2nd. edition CUP) page 76, entry C:

"After if and wish you can use were instead of was (if I were/ I wish it were etc.) So you can say:

1-If I were you, I wouldn't buy that coat. or If I was you I wouldn't buy that coat. 

2- I'd go if it weren't raining or I'd go out if it wasn't raining

3- I wish it were possible or I wish it was possible."

So it depends on the grammarian and/or grammar book you are basing on to justify your choice.

JR