Hi, it does depend on what you are putting between if and was/were:
If I were to leave - good
If it were to rain - good
If I was to leave - good
If it was to rain - good
If you were to leave BUT not if you was to leave
If they were to leave BUT again not if they was to leave
It seems were and was are only interchangeable with 'I' and 'it'
Someone may have differing views; we shall see.
Student or Learner