OED, OAAD, Webster's all say either form is considered correct. Language is a living thing. "Was" was correct before "were" was proscribed by the Latinizers. Now they're deemed equally correct.
It's now accepted widely enough that I generally recommend English learners should avoid the subjunctive entirely: people coming from languages like Spanish are much more likely to over-use it in jarring ways than under-use it, since the few remaining uses of it in English are not even used by many native speakers.
I think I would say "were" in this situation myself, though.
