Traditionally, in 'good' grammatical expression: 'were'... In practice, people very often use: 'was' (which seems to be acceptable now, at least in spoken English. In written English, it is best to stick with 'were' (the 'subjunctive').

"I so wish my father was here" is correct.

This is an example of the second conditional (a hypothetical situation about the present) and the correct form is 'were' (in the second conditional you use the past simple in 'if clauses' or after 'I wish/If only' but the verb 'to be' has always been the exception - it is an example of the subjunctive in English). However, you do hear people say 'was' these days and the British Council website mentions this too.