Hi, Gavin. You’ve given us a good understanding of how to weigh the cost-benefit balance between refined and synthetic engine oils. Thanks for that. However, both are made from industrially extracted and processed chemicals, so why is one called “synthetic” and what are the magic ingredients that make it better? I’m sceptical.
The superior lubricating qualities of so-called “synthetic” engine oil are not about any magic added to it. It stems from what they leave out!
The chemistry of both options is extremely complex, so I view it - apologies to the science of tribology (friction between surfaces) - this way:
Crude oil is a cocktail of many different “elements” (atomically singular substances) that nature has processed into dozens of different molecular hydrocarbon “compounds” (multi-element substances).
Refining separates some groups of those concoctions into seven main types of “product”: gases, spirits, distillates, fuel oils, base oils, greases, and residues. That’s a range from a light vapour to a heavy and gungy tar (mac)!
Unlike most other aspects of life, when it comes to oil, being airy-fairy, light-headed, or a bit thick is not a defect.
The hydrocarbon source is in the DNA of all the different products, so when a base oil is further processed into a specific engine oil, and its specific qualities are further upgraded with a cocktail of additives, it is still not absolutely “pure”. It is selected because most of its compounds are positive for its intended use as an engine lubricant. But some of them are not.
Removing this tiny fraction of “bad guys” might be technically possible, but the process is uneconomic and non-essential. “Natural” modern engine oils are outstandingly good at what they do, so that’s good enough.
Synthetic oils achieve something a little bit nearer absolute perfection by approaching the problem the other way around. Don’t try to get the bad guys out. Just don’t put them in as you source all the good compounds one at a time and make your base oil from them. That, not some man-made substance, is what “synthetic” means in this context. I think.
The result is an engine oil that does not freeze at sub-zero temperatures (because it does not contain the trace substances that do), and by similar omissions, it resists evaporation in extreme heat and gives tribologically better lubrication against friction and wear.
There is no doubt about those superior qualities. The question is what difference they make to performance and/or economy and/or reliability and/or durability in ordinary motoring...set against their extra cost.
The answer is that there is no single computation. In virtually all circumstances top quality synthetic oils will make a difference. But in most circumstances, that benefit will be fractional, while the price difference is multiple.
However, the major price rise applies to the minor retail purchase of a can of oil; the fractional benefit applies to the major capital investment of an engine.
None of the engines I operate on cars, lawnmowers, generators, etc, actually specify or “need” top-quality synthetic oil. But I choose the highest grade of synthetic oil I can find on all of them.
The lawnmower is now 20 years old (and it mows an area of several acres several times every rainy season), the cars and a frequently used portable petrol generator are all 30 years old, and all their original and unopened engines are running quite nicely, thank you.
Then there’s a spoiler. I also have a static diesel generator that lives on very ordinary grades of oil (by today’s standards) that is also running well. That was installed in the farm engine room in 1951 (sic), and for the first 40 years of its life, it was the only source of electricity and was used for several hours every day.
So, it is now 74 years old, runs at about 400 rpm, delivers exactly 240 volts at exactly 50 Hz, and I have no idea what its lifespan might be.
