I'm with Iwombat on this one. My big reservation is not being over-propped and lugging the engine.
I could hit 6700 RPM, (the rev limit) on my 140 looper if I wanted to, so running 5000 for an hour does not concern me at all.
That did not bother me on my old 1976 115 crossflow either, I just used 5800 RPM as the threshhold like Dhadley said to do.