I havent seen a lot of uoa's in the bike world, but I'd imagine they are the same as with cars, as in all miles arent created equal. With 18k miles a year, if the lions share is easy freeway miles, I'd say use a lab to set the interval. I had a 2 year interval on redline with 10k miles on the interval, and I still had over 3 tbn. So as long as you have filtration and everything else the lab determines is ok as in wear, viscosity, moisture and/o contamination, you can push that interval out. Especially in Harleys and Hemi's, I'd rather have high performance oil then fresher group 3 oil, assuming the lab agreed. Of course 5k mile intervals are the most desirable, but if you cant swing that I'd explore my option a little further then just settling on fresh group 3 oil. That is why the number one cult following with redline is the harley world. One of the few places you see redline on the shelf, Harley dealers.
Now that they carry their own oil who knows if they still stock redline, but my local store had a ton if it last time I went. Anyone know what the harley oil is, who blends it?