Is there anything special I need to do to change to syn oil?  Just drain the pan change the filter and put it in?

is 10W30 OK to use?

Is this for car or boat?  If it's a boat I would run the 15W-50, since marine engines are pretty much under a heavy load all the time.  I've been using synthetic in some of my cars (Toyota Tundra and Jeep Wrangler) and my jet boat for more than 10 years.  In a couple of old cars I have, 68 Malibu and 69 Mustang, the synthetic doesn't work as well but those engines are old school builds and I can hear the lifters clatter a little with the synthetic so they get dino oil (if I ever build them again the engines will be run on synthetic).  I've had the Chevy SBC in my boat apart a couple times and having run the synthetic it's very clean inside, no gunk anywhere. I use Mobil 1, because it's been around a long time and also it's readily available at a reasonable price, but I'm sure others will chime in with their favorite.

At least 20 hours on the new pistons/rings/bearings.  These were put in after about 30 hrs on the fresh rebuild.  I replaced hypereutetic with forged pistons.  I put in the Mobile 1 10W30 after talking with Jet boat performance.  They said 20-50 was what they use, but the 10-30 should be fine.  I will replace it with a heavier blend if I sense any problems.  I can't imagine it will be anything but better than the 10-30 regular oil I was running in it...