You should take a look at the top of your motor oil can before buying it.Make sure that the label certifies that the oil meets the manufacturer's requirements and contains detergents for better engine protection.
1)API provides three kind of ratings.
MS Service severe
MM Moderate service
ML light service
Always buy motor oil with MS rating.
2)Look for below statements on the top of can :
Sequence Tested
Passes ASTM sequence tests
Exceeds/meets car manufacturer's warranty requirements.
Exceeds/meets car manufacturer's test requirements.
Exceeds/meets car manufacturer's service requirements.
These statements means that oil performed well under car manufacturer's sequence tests.
3)It should meet the warranty requirements of your manufacturer.
MS rating provided by API and car manufacturer's are different.Every auto company has his own MS rating.You should buy a can on whose top ,"surpasses all new car warranty requirements", is printed.So that in case of engine failures you dont have to pay the single penny.
4) Always buy, High detergent Oil
You will see HD on label.
5)Api ratings for Diesel Engines
DS Diesel service severe
DM Diesel service moderate
DG Diesel service general
6)Difference between "exceeds" and "meets".
The can with "exceed" printed on it will have detergent and dispersant for better engine protection.
7)Muliti Viscosity Vs single viscosity
You should opt for can with multi viscosity rating in the areas where temprature varies.These cans are 15 percent costlier than single viscosity oils but works perfectly on cold start and as well as on providing engine protection at higher tempratures.