Nowadays most ESL exams would accept both. I use 'were' because I was brought upwith it, but don'tmind which my students use. For formal English, 'were'is probably a better choice in the eyes of many and some formal expressions like 'Were he to complete it', 'Were it not for her help...' sound very strange without the subjunctive because they are almost exclusivelyused by people who favour this form.
It's interesting that you say that. I wonder if it depends on the person who is marking the paper. I'm British and I teach business English and maybe that's why I prefer the "If I were you ..." form. When I hear anyone saying "If I was you ..." it's like fingernails down a chalkboard!
I had a teacher (A British one) who accepted "was" for conditionals in the first person singular. I was a bit "shocked" because one day our task was to spot the mistakes in a text and I thought one of them was "If I was ill I'd stay at home". He said it couldn't be considered a mistake because it is used in colloquial English. However, I'd never use it and honestly I still consider it a mistake. Even if I'm not a native speaker... It doesn't make any sense to me!