FTIR

Oxidation - High levels of oxidation by-products are normally the result of high temperatures, overextended oil drain intervals, aeration or even new oil that has been exposed to adverse conditions such as heat or has been stored for extended periods of time. It is a measure of lubricant degradation. They are generally unstable and can deposit on surfaces in the form of sludge. Sludge can form at the point of oil degradation (for example hot spots in the system) or can deposit on other surfaces. They can "cure" to form a hard coating or remain soft similar to a grease-like substance. If left unchecked they can interfere with flow, cause corrosive wear or inhibit heat transfer. Some effects of oxidation residue deposits are sticking valves, plugged oil passages, bearing wear (due to interference with hydrodynamic lubrication), filter plugging and inefficient oil cooler performance. 

Nitration - Nitration by-products are commonly associated with varnish formation. They may also indicate improper air:fuel ratios - especially in natural gas engines. Varnish is also unstable in oil and deposit on surfaces. Varnish is typically a hard, reddish-brown deposits either where the oil degradation has occurred or throughout the system. Varnish can cause sticking valves (especially servo valves) and interfere with heat transfer.