The traditional grammatically correct form is "I wish I were," which employs the subjunctive mood to express hypothetical or contrary-to-fact situations.
Grammatical Explanation:
- The subjunctive mood is used after "wish" to indicate something contrary to current reality.
- "I wish I were taller" (implying that I am not currently tall)
- "I wish I were on vacation" (suggesting I am not currently on vacation)
The subjunctive "were" has historically been the standard form in formal writing across all persons (I/you/he/she/it/we/they).
But nowadays in spoken and informal contexts, "I wish I was" has also become increasingly common and accepted.