In
How to Rebuild Big-Block Ford Engines, Steve Christ says: "Early 427s use the same oiling system as the 390HP. This system feeds
oil to the front main bearing, camshaft and rocker shafts before the remaining main bearings. This means that an
oil-pressure loss anywhere in the lubrication system starves one or all of the the four rear main bearings." This is the so-called top-oiler or center-oiler block. In part, with regard to the sideoiler, he says: "This system ensures that the
main - and
cam-bearing bores receive
oil pressure directly from the oil pump." "If you want the best block for high performance use, the side-oiler is it."
I've got a top-oiler and am generally happy with it; however, the passage above dove-tails pretty well with what I've heard in the past, which is that the side-oiler has a better oil delivery system, and, because of that, it is viewed as a better endurance engine.
TT