Regulations concerning the qualities that engine oils should adhere to, have been around since the beginning of the last century. According to these regulations, it may contain no acids, which e.g., could attack the engine parts. Although, it has been said that there used to be oils, which after not being used for a long time, have broken down into their constituent parts and then have indeed, attacked the engine components. At that time, the temperatures that ensured the viscosity were also specified, the flow-characteristics themselves were pretty vaguely described.
Perhaps in this context, it may be helpful to consider those who handle an oil-change slovenly, or simply don't carry them out. Particularly annoying, is that these people can even produce plausible proof justifying their behaviour, e.g., 140.000 kms without an oil-change and the car is still running, apparently without problems. To offer a solution, we must fall back on not scientifically documented experiences, namely, that over the years, oil gets thicker and thicker.
There are stories in circulation, of veteran cars, where the oil, after a long period of not being used, has had to be scraped out. This does of course, have the advantage, that if only a little oil is topped up from time to time, any leaks will eventually disappear. However, let's be serious, would you trust the lubrication abilities of an oil like this? I think not. Why then, does the slovenly driver feel justified? Because no-one has bothered to take his engine apart to check it for wear and tear. Our engines distinguish themselves through an additional wear and tear prevention, e.g., in the form of a protective coating on the pistons or an additional coating on the bearing cups, the tight-fisted scrooges of course, benefit from these qualities.
If the story about the oil thickening is true, why does it actually happen? There are two factors: firstly, in the course of its operational life, through chemical processes, its viscosity, and secondly, impurities, or simply dirt, also gets into the oil. Thus, e.g., freshly filled oil is, after only one larger tour, almost completely blackened. It has taken in the first soot particles, which the filter can no longer keep out.
Perhaps one can already recognise, that basic oil alone cannot
function very well. The same as when one speaks of alloyed-metals, where their characteristics are improved by mixing them with others, since about the first third of the last century, there have been so-called alloyed oils, which only means, that they contain additives.
In the course of time, more and more additives have been mixed in, nowadays, perhaps as much as 25 percent with only 75 percent basic oil. Only the manufacturers know the exact ratios. One of the first characteristics was e.g., to keep the abrasion- and the oil-carbon particles from the pistons in suspension until they reached the oil filter. Indeed, there are now opinions which maintain that veteran cars without oil filters should be driven with unalloyed oils because the alloyed oils can deposit their load in unsuitable places, perhaps causing bottlenecks in the oil channels.
Who is actually affected by this? You would be surprised, which vehicles have to be taken into account, e.g., the VW-Beetle, more than 25 million vehicles never had an oil filter, except perhaps in the Porsche version. Nevertheless it has always been driven using alloyed oils, after all, where can one buy unalloyed oil?, and what did the Beetle-engine do with its waste? A great deal of it stuck to the valve-covers and in a type of oil sump, which the Beetle never actually had. Defects because of a clogged oil-line?, almost unheard of …
Apart from the characteristic of keeping unwanted particles in suspension, in the 1930s, the additives had other, more important tasks. Beside the protection against wear and corrosion, they were there also to prevent the oil from ageing (oxidation). The third, large factor was the alternation between improved viscosity and temperature behaviour, thus, a relatively easy transportability soon after the cold-starting, and an indestructible oil-film when the engine was hot and peak performance was being demanded.