I read an article once that laid out 2 general rules for synthetic:

1. If you have a low miles motor(~50k or less), switching to synthetic should prolong engine life and seals.
2. If you have more than 50k miles, don't bother switching to synthetic, as seals, by this time, have gotten used to dyno oil, and the composition of synthetic oil can allow oil to seep into the combustion chambers.

I think the second rule is just opinion, though. Changing to synthetic for 1 cycle shouldn't hurt your car, then you can see the difference for yourself. Also, if you just had a rebuild less than 2k miles ago, I would DEFINITELY switch to synthetic.