need advice white scum

my wife's 68 mustang did that for years, It always had a lot of condensation in the aluminum valve covers and nowhere else. I had the block magnafluxed last time I had it in the machine shop... they did not find anything till they were rolling it over to put the cam bearings in and noticed a small crack in a non-stressed place... (after boring it all etc...) they said they have never seen one crack there and it was probably there from the factory. (checking with my father in law, he said it had always had a little condensation in the valve covers... he bought it new in 68...)

As stated above, put a PCV valve on it and take a ride for an hour or two at varying RPM and get it good and warm.... that should help finish seating rings and getting moisture out of it.