The R series are the old school full length MCAT (circa 2004/6 I think) , the 8 hr one. So like Idiaz29 said, they have more questions/passages. In the BS section, you may find some of these extra questions deal with topics that have been since erased from the current MCAT, such as some organic topics, so watch out for those.
Well, if you can find the R exams, there's nothing wrong with answering the questions. The extra practice is helpful and they can include what you would otherwise see on the assessments. Just be aware that, if you don't know a question, it might be because it isn't on your MCAT, rather than because you need to review it.