I'd bet that if you could find an original OM, 10W-30 would have been the recommended oil.
If this engine really does have heavy valve springs, then a little more zinc than is found in most current API spec oils (including dual rated HDEOs) might be in order.
Most 10W-40s contain more ZDDP than current API specs allow for in lighter grades. Defy or Maxlife 10W-40 might be a solid choice.
Just don't go crazy on it if you use ZDDP additives, since too much is detrimental to wear.
I doubt that you'll drive this car enough to get enough use from any synthetic to justify its use and there's nothing magic in any synthetic in reducing wear on short OCIs.
OTOH, since these aren't cheap used cars anymore, to say the least (I can remember when a mid year Corvette was a fairly inexpensive used car), you might want to consult our resident Amsoil guy, Pablo, for recommendations.
Incidentally, you've bought a very neat car.
I am officially envious.
First year for this long-lived generation (the '68-'82 cars were really just re-bodied versions of this chassis) and the ragtop beats the split-window coupe IMHO.
How about some pictures?