Unfortunately, a changeover is not quite that simple. In any case, the engine, whose oil is changed, should be carefully cleaned when changing the different types of oil. The reasons for this are obvious: Fully synthetic engine oils should, according to German legislation, be formulated on the basis of polyalphaolefins (PAO), polyisobutenes (PIB), or a mixture of such and similar synthetically produced hydrocarbons. These raw materials in themselves are similar to mineral oil and therefore also mix well with mineral oil-based lubricants. But additives dissolve slightly less favorably in PAO and PIB, which easily dissolve completely in mineral oil. This problem is cleverly solved by the addition of ester-based synthetic oils (usually diesters and/or polyol esters) or similar "solubilizers". The additives are in fact soluble in these "flux oil" and such in turn in PAO or PIB.
At the same time, PAOs and esters often exert an opposite effect on sealing materials. Some seals shrink when in contact with PAO and PIB, ester oil causes them to swell. If a lubricant contains PAO and ester in equal measure, these effects can balance each other out.
However, when switching from a mineral oil-based lubricant to a fully synthetic oil, the mostly ester-based flux oils can cause difficulties. In addition to the detergent and dispersant calcium- and magnesium-containing additive compounds, the solubilizers impart further detergent, i.e. cleaning, properties to the PAO- and PIB-based synthetic oils. If, as is not unusual for an older engine, there are varnish-like deposits on the temperature-stressed components, e.g. caused by thermal stress and oxidation of the mineral oil previously used, these can be dissolved by the synthetic oil. The particles of these dissolved deposits inevitably remain in the synthetic oil. They can quite quickly coat the bypass filter or affect the transport of oil in the oil wells. Often, such impurities lead to faster oil aging than expected for synthetic oils.
These phenomena often occur when switching from mineral oil-based oils with a lower quality than synthetic oil that meets the latest specifications. The cleaning and flushing effect is often further enhanced by the fact that the latest oils, e.g. SAE 0W-30, are still much "thinner" than mineral oil-based SAE 15W-40 or semi-synthetic SAE 10W-40 oils.
After all, a similar effect can occur not only in engines, but also in gears and in industrial plants when switching from mineral to synthetic oil.