Hi Peter,
I have exactly the same experience. Only, my bike has at least run for about 10000 miles since its rebuild.
When I acquired it about 6 months ago, I changed the oil, and changed it again after 200 miles.
I also noticed the oil was greyish of colour, and on the magnetic sump plug I found a grey fluff which, when I rubbed it between my fingers was like an extremely fine paste.
In my opinion, this can/will do little harm, but if anybody could give a scientific explanation, I will be glad to hear.
regards,
Patrick