What causes the sludge that is sometimes found in the bottom of the oil pan?

Oxidation Resistance

Oxidation resistance is the ability of an oil to resist the direct and indirect attack of oxygen during engine operation. The way in which an oil is formulated determines its ability to resist oxidation. Strong oxidation begins to occur rapidly after the antioxidant additive in the engine oil is exhausted, so the type and amount of antioxidant in the oil determines how long oxidation will be resisted.

Oxidation leads to the formation of acid, deposits, and varnish within the engine which can cause stuck piston rings, scored cylinders, and other engine damage. Oil viscosity can increase with oxidation and, in extreme conditions or with poor engine oil, may cause the oil to gel in the crankcase at normal ambient temperatures.

A much faster way of measuring oxidation resistance than in an engine test is by using ASTM test method D4742. This test measures the time for an oil to reach its breakpoint which is when the oil's oxidation resistance is overcome. When this happens, the oxygen pressure suddenly drops as a result of oxygen chemically attacking the engine oil and forming oxidation products. The time it takes for the breakpoint to occur is recorded in minutes.

There are no specifications for the length of time an oil must perform before showing signs of oxidizing. However, a longer time to reach the breakpoint in this test indicates an engine oil may be more resistant to oxidation — a desirable property.