To elaborate some on what Ron posted. The biggest difference is that the center oiler runs
oil through the cam bearings and to the mains. The side oiler runs the
oil through the mains to the cam bearings. Reversed priorities. I think that under most conditions a center oiler is fine, however under high stress conditions the side oiler is better IMO. Now if you are talking about an original block the center oilers are the oldest and likely the most used up. I also think that the side oiler , besides having a prioritized oiling system also went to "square" cylinder castings for more strength. If you are talking new aftermarket blocks none of that is really appicable since, I think, ALL the new blocks have better castings as well as thicker cylinders, center or side oiler.