That isn't good for parts like camshaft lobes, valve stems, rings or anything else that depends on the volume of oil being flung out from the spinning crank at running RPMs or splash from the rockers.
As the others have said, if you broke it in with synthetic oil..
One thing I thought of that may not apply, bronze valve guides don't do well in motors that get built and then not run...(the guy who owns the local machine shop told me that)
Have you tried wiggling the tops of the valves to see if there is noticeable movement? We recently had to pull the top end off of a 5.0 with alum heads/bronze guides that ran beautifully for several thousand miles, then sat for 7 years while the rest of the car was built. Upon startup, it ran ok, then developed some noises in the valve train...some valves were looser than others, but they were all in need of new guides.
EDIT: just saw your last post..
I'd try dumping the syn oil, and running some regular oil...You are not going to mess anything up more than it already is by doing that. It might not help, but you never know unless you try.