it can be hard to compare same grade of oil from different brands coz all of them may have the same level of certification but they may perform in totally different manners

so for ease of comparison, the spec at the back of the oil is categorized the same for all oils such as viscosity at 40 and viscosity at 100. u will never see oil spec of viscosity at 200 or 65 degree to avoid confusion.

so here are some guides of what i think most crucial when comparing different brand of oils to see which is best suited for ur application:

1) viscosity at 40 and 100 - the lower the number, the thinner the oil, the better the lubrication. from here, we may sometimes see 0w30 can be much thinner than 10w30.

2) Viscosity index - oil ability to behave at rated viscosity at wide range of temperatures. Higher number means higher viscosity stability. important for high heat engine - turbos and high rev NA. lower number means, the viscosity will change a lot from what stated at the back of bottle at different oil temperature than 40 and 100 celcius so it is not good.

3) NOACK volatility - oil weight loss due to evaporation, the number is in percentage. the lower the better. important for turbos coz even branded oil may have high NOACK number which contributes to oil evaporation even in no-leak engines. As oil evaporates, the lighter weight will give up first, leaving the heavier oil so oil becomes thicker and less performance.

4) High temp/high shear viscosity - oil ability to sustain the mentioned condition, the higher the better. most important for turbos and racing engines.

5) Total Base Number - this is the most important feature for all average users like me to see whether the oil can actually be used for long time or not. this is the reserved alkalinity to resist fuel dilution of engine oil from combustion. Fuel dilution makes the oil acidic and therefore oxidized faster. The higher the better. Higher TBN also usually means the oil has lots of detergent which is good for engine cleanliness. Amsoil has the highest as far as i know. So there is no need for rigid classification of fully syn has to be changed at 10k, semi at 7k and so on, look at the TBN and do ur own comparison. Whatever close to 10 is good, higher is best and lower is so-so only. minimum would be 7-8 as dictated by SAE if im not mistaken

To sum up, 0w20 oil may be very thin but if the Viscosity Index and NOACK is really good, u wont experience as much oil loss or bad engine wear as compared to 10w30 having really bad NOACK and Viscosity Index. The specs are there, we just need to learn how to interpret so that we can all stop beating the bush when it comes to what oil is suitable for our application.

For enthusiast/speed chaser:
- pay attention to all specs mentioned above

For average daily drive and normal maintenance:
- pay particular attention to Total Base Number

If the brand of oil you are comparing does not provide any of the info above, it can simply means that the oil did not pass the test or the test result is not impressive that they hide it so people would still buy. We wont be able to say its bad until we see bad test numbers, so hide it and call it good stuff, people will still buy and call it good. Its all about sales in the end

for example, try see castrol oil spec whether they have half of the specs i mentioned above and see amsoil, redline and torco specs on the web, these branded oil have nothing to hide