I'll add only personal comment and I may be off base here..but.
I believe that there are 2 TYPES of oil..alkaloid base and paraffin base.
Each one is built to the label specs,eg.10w40 , 30w ,application , etc.
I have used a paraffin base oil in cast iron engines only and alkaloid base in aluminum engines only.
The reason is , a paraffin base oil reacts with hot aluminum and leaves a yellow film on everything.It acts like a insulator.
the alkaloid base oil keeps the inside squeaky clean.
Yep, mostly off base, sorry. Alkaloid refers to oil based (usually paint) but is not a refining process, nor a type of oil; as it actually just means oil. Paraffin is present in all crude and needs to be refined out to get good oil properties. The simplest way to refine oil is to process it with a clay, a material a lot like kitty litter. The clay will soak up much of the aromatics and sulphur and nitrogen compounds. Then, you dilute the oil with solvent like Methyl-Ethyl-Keytone and/or Toluene (that's the stuff in model airplane glue that's so popular with teenagers), and freeze the oil. The good stuff will mostly stay liquid, and the waxes will solidify and can then be filtered out. This clay-solvent refining process has been around since about 1930.
Oils refined with the clay-solvent process contain a fair amount of paraffin and wax. These molecules cause several problems in an engine: they sometimes fall out of solution, leading to buildups in your engine that must be cleaned out somehow. Also, as these molecules get hot they thin out quite a bit, much more than mineral oil, so they make the oil's high temperature performance rather poor. Finally, at low temperatures the waxes and paraffins thicken the oil so much that you really couldn't call it a lubricant. If you're curious about this, buy a cheap quart of straight 30 weight oil and put it in your refrigerator or freezer over night. You'll be amazed at how thick it becomes. More than half the motor oil sold in N.America as late as 2004 was made from base oils refined with the clay- solvent process, but I don't think this is the type of oil you want to put into an engine you love. These oils are roughly 85% good stuff (oil) and 15% bad stuff (paraffin and wax). To put this in perspective, think of taking a gallon of really excellent oil, and melting a 12" dinner candle into it.
In 1959, Chevron developed a new method of refining base oils called Hydrocracking, where you process the raw oil at high temperatures and pressures with hydrogen and various catalysts. In Hydrocracking, many of the paraffin and wax molecules are broken up into mineral oil molecules, which increases the performance of the base oil dramatically. Also, far more of the aromatics and sulphur and nitrogen compounds are removed from the oil.
Since 1990, Chevron's process has been improved. In 1993, Chevron invented the Hydro-Isomerization process, where wax and paraffin molecules are reshaped into useful lubricants instead of simply being broken up into smaller molecules. By increasing the severity of the hydrocracking process, increasing the temperature and pressure and processing time to process more and more of the unwanted wax and paraffin molecules, the oil's low and high temperature performance and resistance to oxidation can be improved to the point where the distinction between mineral oils and synthetics becomes blurred. Chevron now licenses this process, called Iso- DeWaxing. This process of oil refining is becoming more and more popular, and from about 2004 to the present has accounted for almost half of all base oils. Iso- DeWaxing not only produces much higher-performance oil, but also allows you to start with lower quality crude oil, making us less dependent on the few countries that happen to produce the purest crude oils. This process resulted in what was known as Super Slippery Oils, which were meant to compete directly with Synthetics. It does have the problem tho of having non conforming molecular structure in which the molecules are not uniform in size, and so in high temperature heavily pressured use, this causes the molecules to actually break each other down like large and small rollers in the same bearing would do. This was the reason for formulating synthetics.
Synthetics such as Mobil 1 and Amsoil to name two are formulated from Esters, which are synthesized base oils having uniform molecular structure. Then they are added to with the intent of structuring each seperate type of oil product to work best in a specific application. Esters start life as fatty acids in plants and animals, which are then chemically combined into esters, diesters, and polyesters. Your vegetarian girlfriend should love that. Group V base stocks are the most expensive of all to produce. However, the esters are polar molecules and have very significant solvent properties - an ester base oil all by itself will do a very decent job of keeping your engine clean. So, people who are serious about making a superior oil will usually mix some Group V oils into their base stock.
And finally, the second method of formulating Synthetic lubricant base oils is to start with a chemical called an olefin, and make new molecules by attaching them to each other in long chains, hence "poly." The primary advantage of Poly-Alpha-Olefin "PAO" base oil is that all the molecules in the base oil are pretty much identical, so it's easy to get the base oil to behave exactly as you like. PAOs are called Group IV base oils.
Some people should, in my opinion, clearly use a synthetic oil. You should be using a synthetic if:
you routinely start your engine in temperatures under 40°f, 5°c.
you live somewhere where it gets below -35 degrees, and you want to start your car. In this case you must use either Mobil-1 0w-30 or the Canadian 0w-40 Rotella.
you leave your vehicle sit unused for months at a time.
you are unable or unwilling to change your oil as frequently as every 3000 miles.
you have one of those new 4-stroke MX bikes. These MX bikes hold only about one quart of oil, all of them have marginal cooling systems, and if there's a more severe use of an engine than MX, I don't know what it would be.
That has been oil 101. If you are not interested, you know where the page down button is.