As I know, the adjective of benefit is beneficial. However, I Googled and found so many books and websites use benefitial, which I can't find in any dictionaries like Webster, Oxford, Cambridge or Longman.
I would like to know if there really exists another version that is benefitial. If yes, which reference do you suggest? And if no, what can you say about the benefitial that is being used widely on the Internet?