Answer; “Which ever big-name brand is on special offer!”… was the advice given to me by a senior scientist who works for a particular oil company.
Car enthusiasts (aka ‘Petrol Heads’) will debate for ever over which are the best brands of oil and the usual answers in the UK are Mobil, Castrol, Shell etc. But the easy question to ask these guys is, “What evidence do you have that this oil is best, or even good” and the usual reply tends to be pretty vague because they have no real data.
However, it is worth visiting a US forum called ‘Bob is the Oil Guy’ because there are hundreds of petrol heads there continually debating this issue on that forum and some of them are actually taking samples of oil from their cars periodically and getting them chemically analysed.
The analyses performed on engine oil include;
- Particulates/ suspended solids
- Metals content.
Car engine oils when new are actually (very mildly) alkaline rather than acidic, and become less alkaline and go towards acidity as they age. The degree of alkalinity can be measured and is expressed as a parameter called “Total Base Number” or TBN. Generally, the point at which you are recommended to replace your oil is the point at which the TBN has fallen to one-third of its original value. TBN is a great way of assessing oil condition, but at the moment, you need to pay for the services of chemistry lab like Edotek to get it measured (but is DIY oil testing coming sometime soon????).
The number of particles (made up to some degree by soot from the combustion chambers and metal particles due to wear of the engine) gradually increase with time. The oil filter will take out the bigger ones, but the finest particles get through it.
The change of viscosity of the oil as it ages depends on several factors and may not always be an easy trend to follow.
The top oil companies go to huge effort to blend chemical additives in to their oil to produce a high-performance lubricant. Different companies choose different additive combinations and to a large degree, the merits of modern oils are down to who has formulated the best additive package. If you are tempted to buy a high-quality oil and then go one extra by adding some “magic potion” from the local car accessories shop, then, that is probably a bad idea; you may well be making a good product bad rather than better!
Of course to this point, we have not even touched on the subject of;
- synthetic oils
- mineral oils
The only comment I’ll make in this post is to say that if you research this topic, then what you’ll find is that according to a US legal precedent, the term ‘synthetic’, in the context of engine oil, has no technical meaning and is purely a marketing term. I think that this could be spelled out as meaning that all of the oils on the market are synthetic, but some are more synthetic than others, and tellingly, some cost more than others!