The side oiler was designed for racing, it plumbs oil directly to the crank via a passage molded into the casting's left side, hence the name side oiler. The center oiler or top oiler was a traditional FE design which first oiled the cam and lifters and then the crank. As the pre 1968 side oilers were all solid lifter engines oiling the lifters wasn't needed.

The side oiler was specifically a redesigned FE block for the 427 SOHC or cammer. It was also able to accept any FE heads previously made. In addition to the cammer heads, Medium, High Rise, and Tunnel Port heads were employed to try to get an edge in Grand National (Winston Cup) competition.

The side oiler in addition to the side oiling configuration also were built on a separate assembly line using a hig nickle alloy and tighter tolerances than their FE cousins. They all had screw in freeze plugs and in 1966 were cast with a squared off cylinder mold for greater strength. The side oilers were also 70 pounds heavier than other FE blocks.

I have an FE casting number guide in my photo gallery.