Here is a guide...
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The final number is based upon the kinematic viscosity at 100 Centigrade. Is a rating, how a monograde '40' and a monograde oil '50' behave at tempreature 100 Centigrade.
As you know solid-liquid get thinner when temperature increases. So, the engine oil will eventually 'thin-out' as the engine get hotter. When then oit get thiner to very thin liquid and finally almost as thin as water and the extreme end would be the oil turns gaseous - the oil will loose its 'lubrication' properties (no more behave like engine oil). In simpler words, a 10w50 performs better than 10w40 in hotter temperature.
A small note from Mercedes-Benz Approved Engine Oils for the Latest Engines that might interest you: 5W-30 oil viscosity is not approved for use in all AMG engines thus I don't think any turbo engine or high stress-strained engine should be using 5W-30, must at least use 5W-40 or above.
You should consider what type of motor oil should go into your car. Motor oils are designated by relative weight, which translates to how thick they are and how much heat can they take before breaking down completely. Cars driven in the winter may require a "10w 30" weight oil which won't turn to a thick syrup in the cold. A small car driven in the hot Asia summers might use a "5w 20" weight to avoid overheating. Every car's needs are different, so ask the attendant to select the best weight motor oil for your particular engine. Sometimes it's the same all year round, but in other circumstances it may have to change with the seasons.
If you're planning on taking a long car trip or really want to keep your car in prime running condition, you may want to consider the more advanced oil change options. A basic oil change at a local shop may cover the cost of the oil, a disposal fee and a quick inspection of the car's other fluids and hoses. The next step up may include lubrication of the chassis, tire inspection, topping off other fluids and a more thorough belt inspection. Deluxe oil change services vary from venue to venue, so be sure to ask the manager or attendant about what to expect in return for the higher charges.
How thick is the oil or how it resists flowing?
-1000 grains of oil to flow through a small orifice in the testing apparatus at each temperature.
-The �standard� set for oil from sperm whale was 100 at 70oF