Ed
Possible it was serviced because of a leak or something else on one side and the cause for the clean oil in one side. Some dealers would leave a lot to be desired and only service one side if doing a repair.
Usually the oil does come out black with shiny particles like a metallic paint would look. The 15 wt SE oil you used will firm up the front end quite a bit for you, the stock oil is a 5 wt.that was in there from the factory.
I would just ride it and if you are suspicious of a problem, service it again after putting some miles on it and measure what comes out on both sides and compare the oil in both sides.
The unstable feeling could have been unequal amounts of oil the the two sides before you drained it, I've seen that too.
I have seen people use a wide variety of oils including transmission fluid in front ends.
Here's some info oil the fork oil weights.
Fork Oil Weights - Harley Davidson Community