Definition (Source: Wikipedia)
***Motor oil, or Engine oil is a type of liquid oil used for lubrication by various types of internal combustion engines. While the main function is to lubricate moving parts, motor oil also cleans, inhibits corrosion, and cools the engine by carrying heat away from the moving parts. The majority of motor oils are derived from petroleum.
Synthetic motor oil, consisting of artificially-synthesized compounds, currently has a majority[citation needed] share in the motor oil market place because, while more expensive, it offers enhanced performance.
In engines, there is inevitably some exposure of the oil to products of internal combustion, and microscopic coke particles from black soot accumulate in the oil during operation. Also the rubbing of metal engine parts inevitably produces some microscopic metallic particles from the wearing of the surfaces. Such particles could circulate in the oil and grind against the part surfaces causing erosion and wear. The oil filter removes many of the particles, but eventually the oil filter becomes clogged. The motor oil and especially the additives also undergo thermal and mechanical degradation. For these reasons, the oil and the oil filter need to be periodically replaced.
The vehicle manufacturer specifies which SAE viscosity grade of oil should be used for the vehicles it produces but many different weights can actually be used. Some manufacturers have specifc quality test requirements or "specs" for service in their particular make. Quick oil change shop's recommended intervals have been 3,000 miles or every 3 months, but one oil brand now makets miles-specific line with 5000, 7500 and even 15,000 miles performance guarantee.
With a degree of ambiguity about how many miles motor oil is actually good for, some people opt for more convenient time-based schedule. Seasonal changes are desirable where the viscosity can be adjusted for the ambient temperature change. Also, time-based intervals account for both the short trip driver who does less miles, but builds up more contaminates, as well as the long highway trips that are much easier on the oil. Many modern cars now list somewhat higher intervals for changing of oil and filter, with the constraint of "severe" service requiring more frequent changes with less-than ideal driving. Short trips of under 10 miles, where the oil does not get to full operating temps long enough to burn off condensation, excess fuel and other contamination that leads to "sludge", "varnish" or other deposits. Many manufacturers have engine computer calculations to estimate the oil's condition based on the factors which degrade it such as rpms, temps, trip length and one system adds an optical sensor for determining the clarity of the oil in the engine. These systems are commonly known as Oil Life Monitors or OLMs.