When we work with FE's, we're working with architecture and technology from the late 50's and 60's.
They sometimes have leaks.
The only real thing in an FE's favor is the one-piece
oil pan gasket. Other than that, the 4 piece rear main seal, valve cover gaskets that cover a seam, etc., aren't really the epitome of sealing.
The use of an aluminum block exacerbates the chances of those leaks. The block squirms around about .010-.015" with heat expansion and cooling. It's sometimes very hard to keep everything sealed up over time.