Formal writing in online instructions

I'll second Christy's recommendation of checking out Patti Shank's work. She's fabulous at taking the latest learning research and translating what it actually means in the real world.

Also, this article mentions research on the benefits of conversational writing for learning. The writer sourced the research info from the book Elearning and the Science of Instruction by Ruth Colvin Clark and Richard Mayer, so if you can get your hands on that it might have even more research you can pull from.