An engine oil is basically base oil blended with additives.
While simple, the makeup/quality of these two ingredients will determine the performance/quality of the engine oil in question.
Therefore, performance/quality is dependent on:
1. Basestock used (synthetic, mineral)
2. Additives used
1. Basestock
The kind of basestock used will determine whether the EO is synthetic or mineral, i.e. if synthetic basestock is used then the EO will be fully-synth. However do note that there are different "grades" of synthetic basestocks, for e.g. PAO (polyalpholefins) and Group III basestocks. PAO is of better quality since it is totally not derived from crude oil (man-made in labs) but costs much more to produce than Grp III basestocks.
There are not many brands of EOs on the market that uses PAO as the basestock due to the higher cost of production, and the general public's ignorance on the difference between PAO and Grp III synthetic EOs. Most EOs on the market (including those produced by oil majors) currently are Grp III-based but selling at PAO prices.
Without naming any brand for impartiality's sake, you might want to check with your mechanic which brands are PAO and which are Grp III based. Usually the price difference wouldn't differ by a lot. To be doubly-sure you can go check out the Product Data Sheet (PDS) from the manufacturer's website.
2. Additives
Additives are simply chemicals that are added to the basestock to impart or enhance certain qualities of the EO, for e.g. detergency effect, emulsifying effect etc. Do note that there is no one additive that can do everything, i.e. be the best in everything because the effects are generally opposing each other. For e.g. adding too much additives that increase the performance of one property will weaken another. Henceforth the key here is to find one that has a balanced additive pack that is formulated for the application of the EO.
How come some fully-synth are so cheap:
"Standard" additive packs can be easily purchased from additive manufacters. Same goes for basestock. So what happens is that anyone with enough capital can simply register and set up a company that purchases the basestock and standard additive packs, blend them and sell to the consumers (which is what a lot of those smaller EO brands are). In short, to use electronics sector as an analogy, they are not the OEM manufacturer but are simply "assemblers", buying generic parts and assembling them together, package it to their brand and market it to the end user.
And that is why you see a huge price variance between so many brands of Engine Oils.
Industry Standards:
A little fact that few people know of is that the SAE/API specifications for EOs is on a "self-declaration" basis. What that means is that the SAE/API do not require manufacturers to send in batch samples for testing to validate whatever SAE/API specs they claim their products to possess. Therefore, there exists the risk of misinformation. Logics goes that it is generally safer to go for a reputable brand since they got the company reputation to protect.
However, certain OEM specs like for EOs specified for use with Mercedes engines (for e.g. MBxxx), or Volvo engines (for e.g. VW502.00 etc) requires the manufacturer to send in samples for certification. This process requires the manufacturer to sometimes invest heavily in R&D with the engine manufacturer to produce a compatible oil, and the tests needed to certify the EOs is costly. Henceforth usually only reputable brands have such OEM claims.
Hence, why some brands of EOs are more exp is because they use better quality ingredients (PAO basestock, specially formulated additive packs) and invest in R&D to come up with the special formulation, and also to subject the product to testing and certification which "generic" manufacturers don't.
Conclusion:
Quality/Performance of an EO is dependent on:
1. Quality of the basestock used (PAO vs Grp III for synth, or mineral)
2. Kind of additives used (there is no industry standards for this, but general rule of thumb is better EOs use specially formulated additive packs instead of generic additive packs, but of course are much more exp)
NOTE: I've seen and heard many people commenting on the suitability/compatibility/performance of an EO simply based on its viscosity. For e.g common argument goes something like "0W40 is too thick, 0W30 is better cos its thinner etc". Pls note that this is not the sole/main determinant in EO performance. The other factors listed above are just as, if not more important in determining the EO performance.
