What is a syn­thetic oil?

Car Mechanics (UK) - - Engine Oil & Lubrication -

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.

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