If I post a comment on a business page, like Dr. Oz, will my friends see it without going to that page? Or will they be notified of it? I wanted to post on a blog there, about a rather personal issue, and I don't really care if people I don't know see it, or even certain people in my friends list, but there are people in my friends list I wouldn't want to see it. Is there a way to avoid this?
Short answer: Yes, your friends will be able to see it
Long answer: On you own wall you decide the privacy, on a business page everyone visiting would be able to see the post plus your activity would be part of the activity feed shown to your friends, which the might or might not notice.
Try visit your activity log on your homepage, there will be a globe for these types of global posting.
work around which I believe will be more and more popular. Make and alternate account for this sort of stuff, technically not allowed. Should be done using browser in private mode, to be sure you don't accidentally reveal the alternate alias.