I will be recording my grandpa telling his life story in a couple weeks. ...what setup should I use for this?
For oral histories I'm very partial to doing binaural recordings. A bunch of universities and museums (our customers) also use binaural mics for their oral histories.
It couldn't be simpler: mount one mic near each ear of your ears, set levels and hit record. Played back over headphones, it'll be a very realistic "you are there" recording.
Even the lowest cost binaurals will do the job.
Alternately, you can use a binaural mic set as two lavalier mics, with one clipped to the interviewer's lapel and the other to the interviewee's. You'll each end up primarily in one channel, and then you can handle the mix in post-production. Recommended if you prefer stereo to binaural.