yes...

1) is 20W50 the mfgrs. recommended oil for this engine?  If not, did
you build the engine with looser clearances that would indicate going
to a heavier oil?  If not, i'd stick with the mfgrs. recommended
weight, (although if that is a 5W20 I would seriously consider using a
5W or 10W30 - 20 weight oil just seems to thin for good protection to
me.  I know, I don't really have anything to back that up, but a 30
weight is as thick as most newer engines are designed for, with a few
exceptions.)

2) No reason not to go to synthetic, but I'd wait until the engine has
some miles on it.  How many really depends on how long it takes for it
to feel broken in.  If it uses oil at first and at some point stops
using any oil at all, after that point I'd feel safe switching to a
synthetic.

good luck,

nate

Somewhere in the dark ages past, word got around that if you used
synthetic oil from the start it would be SOOOO slippery that the rings
would never properly seat and you'd have a chronic oil burner.

It might or might not have been true, to some degree. But the Corvette,
Viper, and a number of other cars come from the factory with Mobil 1. I
switched my freshly built Mopar 440 to Mobil 1 at 1000 miles. I did the
initial fire-up on Castrol 10w30 (20w50 is for worn-out engines, not new
ones- WAY too thick!). Did the cam break-in run for 20 minutes, changed
the oil and filter to get all the break-in lubes and any leftover grits
out. Then ran for 100 miles on 10w30, changed oil and filter again. Then
at 1k miles, I switched it to Mobil 1 10w30. So far its got about 15,000
miles on it, oil consumption has dropped to nil (or as close to nil as
you can get with a big-bore v8), and its running fine.

There's no good reason.  Until conventional oil breaks down, its properties
are the same as synth.  The only reason for using dino oil is the cost
of filling and tossing synth.  Use cheap dino for 50, 500, and 3000, and then
switch to synth every 10-20K.