Were is the singular Subjunctive form (I were, you were, it, etc, were).
It would be best to use this form when writing of an action or condition that is not positively so.
In current usage, the Subjunctive is not used as much as it once was. But it is, all the same, correct to use the subjunctive when writing of anything uncertain.
If only she were here...
If only she was here....
may be acceptable these days. But I think it best to be as correct as reasonably possible, so I would use the first example. It is hardly ever wrong to be correct.