a4tdi, I've bought 3 new Harleys, 2 were Sportsters. I still have 2 of the 3 (1XL, 1FL) and I still don't know if there are all the way broken in. They seem to get smoother and "looser" (in a good way-as in revs up faster) as time goes by. My RK definietely felt different after returning from Sturgis last year, having about 7K+ when I got back. I took my '96 Sportster to Sturgis in '96 with the about the same starting mileage (about 3K) and returning with about 7K and noticed the same thing-smoother running. Even 11 years later it seems like the bike stills gets a little smoother as the years go by. As far as when they are broken in enough to use synthetic, I'd say very quickly, within 500 miles IMHO. Broken in enough to see a decrease in oil consumption is usually less than 5K, often less than 3K. I believe the official "considered fully broken in" is usually about 3K but from some oil studies done, wear on some engines (automotive) doesn't come down to "normal" levels until 30K+ on some vehicles while it only takes a few thousand on others.
I assume that you are asking because you are thinking about putting synthetic in now. I say go for it. I put it in my RK at 453 miles with no problems at all and no consumption since I switched.
On my RK, when I did a used oil analysis after Sturgis (approx. 7K) most of the wear metals showed lower than average, which I thought was pretty good for a "new" engine that was subjected to about 15 dyno runs, 100+MPH travel and a lot of stop-and-go in 90F+ temps.
