Oil viscosity, or thickness, varies considerably by temperature. Because of the operating temperature ranges of automotive internal combustion engines, a single-viscosity oil cannot be used as it would be too thick when cold or too thin when hot. Thus almost all vehicle engines use multi weight oil.
Generally these multi-weight oils are created by single weight oils treated with viscosity index improvers (VII). The goal is to have oil with the viscosity of the base grade when cold and the viscosity of the second grade when hot. This creates an 'all-season' oil for variable temperatures. The viscosity of multi-grade oil will still vary logarithmically with temperature changes, but the change is lessened.
Oil weight, or viscosity, refers to the thickness. The temperature requirements set for oil by the SAE (Society of Automotive Engineers) is zero degrees F for low and 212 degrees F (100 degrees C) for high.
Oils meeting the SAE's low temperature requirements have a 'W' (for 'winter') after the viscosity number/rating (for example 10W), and oils that meet the high ratings have no letter (for example 30).
An oil gets this number rating for viscosity by heating it to a specified temperature and then allowing it to flow out of a constant-sized hole. The viscosity rating is then determined by the length of time it takes a standard oil quantity to flow out of this hole. Quicker flows get a lower rating, slower flows get a higher rating. So larger numbers are thicker.
So oils are tested against the 0 or 210 degree F rating set by the SAE, for speed, as they flow through a constantly sized hole. Thus a 10W30 rated oil flows at a rating of '10' at zero degrees and a rating of '30' at 212 degrees. These numbers are merely a relative SAE specification number and not an actual viscosity measurement.
It is important to note that other oils such as gear oil use a different SAE specification and thus 90W-140 gear oil does not necessarily have a higher viscosity than engine oil.
Oil pressure results from resistance to flow, it does not indicate how much oil is flowing or where it is going. Thus the oil pressure in an engine indicates generally how pressurized the oil film is in the clearances between moving parts.
This is why even low oil pressures are acceptable, as it merely indicates that the oil is present in the clearances.
High oil pressures can be very bad. If the engine clearances are over pressurized, that excess pressure will start to look for the easiest path to escape. This easiest path may not be the direction it is desired to go, which is through the oil galleries to the next series of parts that require oil. If too much oil escapes between clearances, the oil volume further down the line can be considerably reduced.
For this reason engineers try to keep oil pressures within a range where there is enough pressure to maintain an oil film between moving parts but not too much that oil escapes excessively between those moving parts.
An excessively high oil pressure can also indicate that there is too much resistance to flow within the system, meaning the oil is not moving fast enough to properly cool moving parts and replace itself as it moves through the system. Thus an oil with too high a viscosity can damage an engine significantly while still indicating a high oil pressure. This is another reason that for diagnostic purposes, oil temperature is an equally important metric. If the engine is overall running hotter the oil may tend to run hotter.
Most oil pumps maintain a set pressure range by having an internal bypass, usually spring-controlled. When the pressure exceeds the spring resistance, the bypass opens up and allows excess oil pressure to return to the sump without going through the pump and entering the pressurized galleries inside the engine.
Generally higher pressure oil pumps are available for most common engines. The higher pressure is obtained usually by changing the pressure bypass spring to resist a higher pressure, thus raising the pressure in the system.
Higher oil pressures than specified by the original engine manufacturer most likely have no benefit, and may reduce the system efficiency. A common metric some mechanics use is approximately 6 PSI (pounds per square inch) per 1000 RPM. Higher engine speeds put more force on the oil film between some parts, requiring a higher pressure to maintain the film thickness as a result.
Thus observing a 'low' oil pressure in the system at low engine RPM is not usually a bad thing. Low oil pressure at high RPM is a bad thing though, indicating that the pressure does not change with pump speed. This usually indicates a problem with the pump itself not being able to maintain enough pressure.
Where in the system the oil pressure is measured should not have an affect on how it is observed. Generally engines are designed so that the highest necessary volumes are in the front of the system, and the lower necessary volumes come later. This is why high-impact locations like crankshaft journals, piston rod journals, and camshafts are at the beginning of the system. Valvetrain components that require a less volumized oil film are usually saved for last as the system volume may be lower the farther through the system it goes and loses some minor pressurization to the escape of oil between moving parts. As a whole, a normally functioning system should register roughly the same pressure.
So different engines may have minor differences in the observed oil pressures by where the gage is placed. On big block Chevy engines, for example, the oil pressure take-off is directly after the pump before oil passes into the filter and through the system. Small block Chevy engines generally measure their oil pressure at the top of the engine after the camshaft journals. However regardless of where it is measured, there should not be significant drop in pressure from start to finish as this can indicate significant oil loss at some point between.
Oil volume is a metric that is rarely measured in engine parameters. It is preset by the size of the pump gears (in a mechanical system). Volume becomes more important the looser the engine clearances are. Looser clearances and wider gaps allow more oil to escape between parts as it travels through the system, and a higher volume is required to be pumped initially to make up for this loss.
Often race engines which use looser clearances for less friction will require a higher volume pump to make up for the oil loss as it travels between parts. A normal factory clearanced engine should never require a high volume pump. If it does, this is an indication of extreme wear that should probably be addressed in other ways such as a rebuild to restore the factory clearances.
While high volume pumps are available on the aftermarket, it is likely that using one in a standard engine will not add any gains to the system, and indeed may hurt power. Higher volume pumps generally require more energy to run, resulting in an overall loss where no excess volume is necessary.
Altering the oil volume outside of factory specifications usually only results in more oil being taken up by the pump, and more oil returned directly through the bypass. It does not change the overall volume in the system.
Using a high volume pump may make the installer feel better, but overall it is not a good idea where one is not specifically required. These pumps can make up for deficiencies like loose tolerances (or the necessity of loose tolerances), and are good at that job however.
One example of a necessary application of high volume oil pumps is in fast and high revving engines where the standard pump does not have the volume to quickly infuse the necessary oil into the system under extreme and fast load/speed changes. In this case the excess volume/pressure is used to make up for the mechanical pump's inability to react instantly to internal changes. It gives the pump an operational 'cushion'.
Mechanical pumps do an exceptional job of supplying the correct volume and pressure based on engine speed under most conditions. While electric pumps can be useful (especially in dry-sump or racing applications), mechanical pumps are the standard for reliability and efficiency.
Obviously the multi-weight oils used in most engine applications are designed to run in a range of temperatures. As long as the operating parameters stay within these temperature ranges then all is well. However if engine temperature falls outside this range then the oil loses effectiveness.
Engines are designed to run a certain weight oil that will maintain the operating clearances and oil films on the parts at the temperature range the engine is designed to run. Obviosuly the engine starts at ambient temperature and rises slowly to operating temperature. The ambient temperature is the low measurement of the oil weight, and the operating temperature is indicated by the high measurement.
The SAE uses measurements that define these two ends at zero degrees F and 212 degrees F. So in general terms, when using the correct specified weight oil, as long as internal engine temperatures stay between within this range all is well.
The 'operating' temperature is not the same as the 'internal' temperature, as the operating temperature is usually defined as the average coolant temperature in the water jacket of the engine. Most engines run at about 195 degrees water jacket temperature, but in reality a hotter internal temperature.
Most engines are designed so that the oil is cooled by the water jacket through heat transferance. The oil removes heat from the internal parts as it passes, and this heats up the oil. The heat from the oil then passes to the metal around the sump or the metal between the oil and the water jacket, then finally to the coolant or ambient air rather than directly being transferred out of the oil.
Engines that require a dedicated oil cooler pass the oil directly through its own cooler/radiator to remove heat without the double transfer of first having to pass into the engine coolant or metal. It is more efficient, but requires more parts and cost which is why it is less common. These systems are generally reserved for engines that will have a higher internal heat load due to higher constant RPM use.
In general engineers try to design standard engines such that the heat from the oil can be transferred in the less efficient manner. While this lowers the potential operating conditions of the engine (such as extended high RPM use), they are designing around the assumption that most users will not require it. This is why most tow packages on vehicles require some sort of dedicated oil cooler. The high load conditions introduced by towing usually require the engine to do more work and operate in a higher RPM band.
We have heard a general opinion that it is good to have higher oil temperatures, for the reason that it removes water or water-based contaminants from the system being so hot. In reality any sufficiently long period of heated use is enough to do this as water will evaporate and remove itself under even extended warm conditions. In reality oil temperatures should always best remain at or below the 212 degree F range used to rate the oil.
In general if the oil temperature runs close with the operating temperature, then the system is balanced and neither the coolant or oil is carrying more heat burden than is necessary. Obviosuly there are exceptions, but this is a general rule-of-thumb. Generally the oil temperature may be slightly higher than the operating temperature due to the intimacy of the oil with moving parts.