In hypothetical conditional clauses, i.e. those expressing an unlikely or imaginary situation, either the hypothetical past (was) or the past subjunctive (were) may be used. The past subjunctive were is more formal and likelier to be found in formal writing, whereas the hypothetical past was is more common in speech.
The past subjunctive is, however, generally used in American English, which in many ways (e.g. grammar, punctuation) is more conservative than British English.
Likewise, where American English would have We suggest that he leave soon, British English is more likely to have We suggest that he leaves soon or We suggest that he should leave soon.