You get "most of it out" so there is less of a chance for a problem.Why get 'most of it out' so you only have a little problem? If there's going to be a problem I'd think it would be between the pump and the engine, which I highly doubt there would be a problem in the first place.
This question comes up 10 times a year, and I've never heard "well, that didn't work out".
I started with Doo oil, went to Mystic, Klotz, a quart of gas station crap and back to Mystic without ever a problem. If there's going to be a chemical reaction so it jells, you better have every drop out. So 'a little or a lot' makes no difference.Now I'm wishing I emptied my oil res before this trip lol....I've been using the same Mystik Synthetic, just the possible junk at the bottom.
It did flow good when I was getting the BigWorm pump mod and reinstalled it.