Grawgin

Hey guys! So i'm in the middle of 4l60e swap and the "good" transmission has
what looks like black sludge in it. I've read other forums and im getting mixed reviews on weather or not i should put it in.
from what i understand.
Clutch dust=normal
Metal debris=BAD
I ran a magnet slightly above the sludge but didn't retrieve any metal. So I just don't know.

It came out of a truck with 300k highway miles.
Fluid was older than 80k
It didn't smell "burnt" like the original tranny.
and the guy swears up and down it works.

what do you guys think? should I clean it up, throw a qt of Lucas conditioner in and cross my fingers?
Im on a budget and this tranny was 100$. so what should i do? I've never done this before.