See Figures 1 and 2
Use only quality oils. Never use straight mineral or non-detergent oils, that is, oils not equipped with special cleaning agents. You must not only choose the grade of oil, but the viscosity number. Viscosity refers to the thickness of the oil. It's actually measured by how rapidly it flows though a hole of calibrated size. Thicker oil flows more slowly and has higher viscosity numbers, such as SAE 40 or 50. Thinner oil flows more easily and has lower numbers, such as SAE 10 or 20.
Mitsubishi recommends the use of what are called " multigrade" oils. These are specially formulated to change their viscosity with a change in temperature, unlike straight grade oils. The oils are designated by the use of two numbers, the first referring to the thickness of the oil, relative to straight mineral oils, at a low temperature such as 0°F (-18°C). The second number refers to the thickness, also relative to straight mineral oils, at high temperatures typical of highway driving (200°F (93°C)). These numbers are preceded by the designation "SAE", representing the Society of Automotive Engineers which sets the viscosity standards. For example, use of an SAE 10W-40 oil would give nearly ideal engine operation under almost all operating conditions. The oil would be as thin as a straight 10 weight oil at cold cranking temperatures, and as thick as a straight 40 weight oil at hot running conditions.
Note that diesel engines require different oils. This is because the oil gets much hotter, due to running with the high compression rates used in diesel engines. Be careful to adhere to all recommendations strictly for the longest possible engine life and best service. Oil recommendations are as follows.
NORMALLY ASPIRATED ENGINES
5W-20 is not recommended for sustained high speed operation regardless of the weather.
5W-30 oils should be used only in extremely cold areas, where the temperature is consistently below freezing when starting. 5W oils are not recommended for sustained high speed or high rpm driving.
There are excellent synthetic and fuel-efficient oils available that, under the right circumstances, can help provide better fuel mileage and better engine protection. However, these advantages come at a price, which can be more than the cost per quart of conventional motor oils.
Before pouring any synthetic oils into your vehicle's engine, you should consider the condition of the engine and the type of driving you do. Also, check the manufacturer's warranty conditions regarding the use of synthetics.
Generally, it is best to avoid the use of synthetic oil in both brand new and older, high mileage engines. New engines require a proper break-in, and the synthetics are so slippery that they can hinder this. Most manufacturers recommend that you wait at least 5,000 miles before switching to a synthetic oil. Conversely, older engines are looser and tend to use more oil. Synthetics will slip past worn parts more readily than regular oil. If your truck already leaks oil (due to worn parts and bad seals or gaskets), it will leak more with a slippery synthetic inside.
Consider your type of driving. If most of your accumulated mileage is on the highway at higher, steadier speeds, a synthetic oil will reduce friction and probably help deliver fuel mileage. Under such ideal highway conditions, the oil change interval can be extended, as long as the oil filter will operate effectively for the extended life of the oil. If the filter can't do its job for this extended period, dirt and sludge will build up in your engine's crankcase, sump, oil pump and lines, no matter what type of oil is used. If using synthetic oil in this manner, you should continue to change the oil filter at the recommended intervals.
Trucks used under harder, stop-and-go, short hop circumstances should always be serviced more frequently, and for these trucks, synthetic oil may not be a wise investment. Because of the necessary shorter change interval needed for this type of driving, you cannot take advantage of the long recommended change interval of most synthetic oils.