If one would examine the oil from the last oil-change, e.g., for metal debris, it would be possible to draw certain conclusions concerning the wear and tear which had taken place in the engine. Indeed, engine oil has the important job of picking up the, mostly metallic debris caused by friction, and to transport it to a suitable place, e.g., the oil-filter, where it can be stored without causing damage.
Of course, the foreign particles don't all come from friction in the pistons or bearings. It must also be considered whether the oil contains an additive against abrasion or not. Fine dust particles in the air and other solid substances, which are only developed inside the engine through chemical- or physical reactions (e.g. soot) can also find their way into the oil.
Should the particles in the oil be smaller than the pores in the filter-paper, it's time for an oil change. Furthermore, the new engine oils are alkali (in contrast to acidic), to neutralise the metal attacking acids which are formed in the engine. Whereby, in time they lose this ability.
This also means, that one could check the engine oil to ascertain whether an oil-change is necessary. This test however, would be somewhat more complicated than, eg., testing the brake fluid. This testing, by the way, is done in ships engines, which have as much as 20 tons or more oil in circulation, also, generally only a portion of the oil is replaced.
What the oil should also not do, is build up foam. This can happen if the oil level is to high, and the crankshaft splashes through the oil. Foam consists partly of air, thus its development endangers the oil pressure. In addition, there are also engine additives which reduce the build-up of foam.
The particular density or viscosity of an oil alone is no indication of its quality. The ratio of base oil to the additives is approx. 80:20, whereby, the base-oil is also no longer pure natural oil. Already in the production process, care is taken to develop multi-range properties. Radically changed base-oils are used in the production of synthetic oils.
In the viscosity information, the two values before and after the 'W', are formed by measuring the kinematic viscosity at 40°C and at 100°C. Before the oils are classified into the various SAE (Society of American Engineers) classes, the results of the dynamic vicosity measuring are also considered.
Similar to petrol, cleansing additives are also mixed into the engine oils. Thus e.g., no solid components should deposit themselves in the space behind the piston rings, this is necessary for the additional contact force applied through gas pressure. The stickyness of the piston rings is in fact, also the beginning of engine damage.
Therefore it's better, when metal parts which appear inside the engines remain as shiny as possible, indeed, this is not realistic in the case of the piston crown or the gudgeon-pin area. If however, shiny metal is exposed, it can be attacked by aggressive products which are formed during combustion.
In such cases the protective
function of the oil is required. It is also interesting that in this context, a gas, e.g., nitrogen, which meets with the engine oil, can lead to oil-sludge. Oxygen can also thicken the oil. Once again a reason for an oil-change, in any event, also in the case of a creeping loss of viscosity.
The water content of an oil seems to be quite significant. By the way, the technical term for water mixing with oil is sometimes called 'emulsion'. Should water containing glycol be found in the oil, this is an indication of a real defect in the cooling system.
Water in the oil, as opposed to fuel in the oil, can be easily detected by dripping the oil onto a paper filter. If it comes from the combustion or the combustion chamber, it must escape through the crankcase ventilation, which can, e.g., with the development of ice, cause a blockage and thus engine damage. The oil moves away more and more of the from petroleum distilled base product by additives and of course also of plant oil. This would indeed contain less sulfur, but is not suitable for modern engines The industry takes rather the opposite way, ie from the natural oil with corresponding molecules to design the oil from scratch and names it accordingly synthetic oil. It is not least thanks to it that the change intervals could be significantly extended in the meantime. 11/11