What is a synthetic oil?
The majority of engine oils that are designed for cars made within the last 15 years are fully-synthetic. Therefore, debating the use of a mineral oil, semisynthetic or fully-synthetic has become less relevant for today’s motorists.
However, for owners of older modern cars and some classics, the issue remains important. In simple terms, a synthetic oil is man-made from elements of mineral oil, which offers superior lubricating qualities, especially under higher temperatures. In general, semi-synthetic lubricants offer superior protection than a mineral multigrade, but checking the ACEA/ API provided on the bottle might provide a clearer answer. An even harder decision is whether, or not, you should use a fully-synthetic race-type oil (such as one specified for a motorcycle engine) in a classic or retro car that was intended to run on a mineral lubricant when new.
Due to the absence of research and empirical data, many engineers could not give us a concrete answer about whether, or not, this would be a bad idea. Generally, it was accepted that the oil would be more likely to stay in grade in high-temperature conditions, but uncertainty was expressed if old oil seals would be compatible with the modern chemicals.
While Shell advises caution in switching from mineral oil to a fully-synthetic oil, due to its higher ‘cleaning power’, it advises that this should not pose a problem with an engine that has either been overhauled, or is known to be clean inside.