1. How many miles between oil changes with synthetic?
I run about 4,000, but have gone 6,000.
Now, this is on full-synthetic MC-specific Mobil-1 V-Twin and Amsoil MCV.
The other synthetics that I've tried (Several automotive and one MC-specific) did not hold their viscosity even at 2,000 miles, so I can not make an extended mileage recommendation on anything but the Mobil-1 and Amsoil products.
2. I have 1,500 miles since my last oil change. Normally I would wait to switch but I have an opportunity to have an oil change done at a shop, using synthetic oil, for $60. We are getting special pricing for members of our riding group. Would is be worth it to just get it done with only 1,500 miles at this price?
I do it myself for $50, that's using a Mobil-1 filter (M1-108 will fit the 900).
What oil are they planning to use? If anything other than M1 V-Twin, M1 Racing-4T, or Amsoil MCV or MCF, I'd do it myself. $60 isn't a bad deal if that includes labor and a quality filter (stay away from Fram/Pennzoil).
3. What positive changes can I expect with synthetic vs. regular oil?
Nothing glaringly obvious when riding.
Some people report smoother shifting, some don't. Some report reduced temperatures, some don't.
What I can guarantee is that you will get better cold-flow performance (better protection on cold starts), higher resistance to coking which will reduce sludge and varnish buildup, and better viscosity stability IF it is a good quality synthetic.
My bike takes 3 1/4 quarts for an oil change.
I think it's closer to 3.5
Here's my personal test results:
All but the V-Twin and MCV sheared down from 50wt to 30wt in 2,000 miles, including the automotive synthetics, which actually didn't do any better than Castrol GTX.
The two above oils had barely broken into the 40wt range after 6,000 miles.
I would not run a non-synthetic longer than 2,000 miles.