Here is a picture I took of a connecting rod that came out of a Simplicity riding mower with a 14hp Briggs and Stratton. I replaced this connecting rod with a new one because the owner ran the motor too low on oil. Notice how the metal discolors, and becomes rough to the texture. Once the metal begins to overheat, the volatile properties in the oil get used up. As a result, the oil sticks to the engine parts as a tar-like paste. As the metal becomes rough, excess friction is produced between parts. If ran to long, parts can either weld together or break into pieces. This particular rod started coming apart, causing to much slack between it and the between the crank pin, which in turn made a loud knocking noise when the engine ran. In the pictures below, I wanted to give you an idea of the difference in texture and appearance from the one I removed, compared to a new one. Notice how smooth and clean the metal is on the inside of the connecting rod in the bottom picture. Though this picture is of a new rod, a good used one will look similar.
1-oil flow port- this allows the oil to access the crankshaft journal.
2-dark tar-like oil that has stuck in the scratches of the aluminum.
3-rough score marks that have etched the connecting rod end cap