I'm getting ready to do the break in of a new cam I installed in my exisitng 400. Do I need to drop the oil after the 20 min. break in or do I change it after 500 miles. As well, I'm adding some Lucas break in additive with this first go but do I need to keep using it after this and for how long?

You are not going to hurt anything by changing it, if it were me I'd probably run it for a short while and then change it. If you are running a flat tappet cam you are going to want some additive with zinc and phosphorus or run a quality oil with the zinc already in it. I would suggest Valvoline VR-1

Yes, change the oil after you run it for 20-30 minutes. Then change oil/filter again at 500 miles. Run something better than lucas additive. I run GM EOS. My local gm/chevy dealer still sells the "old style" EOS in a round bottle(at least they still did last summer) which is superior to the EPA mandated stuff they get in the USA now in an oil quart shaped bottle. I followed the instructions that came with my cam and ran it for 20-30 minutes varying the rpm between 2500-3000 by about 200rpm every minute or two. Just did this last summer and no cam failure yet.

A second for using VR1 oil for break in. Did this with a flat tappet cam last year and no worries to date. Even added a bottle of STP blue as well JUST for a lil more .