Bharat Stage Emission Standards(BSES) commonly known as BS. It is an emission standard established in 2000 by the Indian government to regulate the output of air pollutants from combustion engines.
Govt. has recently announced that the BS4 engines would not be sold in the market after March 2020. And only BS6 would be sold after that.
The diesel engines show a big difference in comparison to petrol since the diesel will release less Particulate Matter(PM) which will cut down nitrogen oxides by 70% and in Petrol it is 25% so, buying a BS6 over a BS4 could be a better choice if you would want to save the environment. The BS6 engines would be lowering the mileage of the car as well.
The BS4 diesel contains 50 Parts Per Million(PPM) sulphur and BS6 will contain only 10 PPM sulphur so, filling BS4 diesel in BS6 engine could harm the engine in the long run and vice versa. But filling BS6 diesel in BS4 engine would cut down its emission as well because of lower sulphur in the fuel. In a Petrol engine, the difference is not much so you can use BS4 fuel in the BS6 engine and vice versa without worrying about anything.
Indian Oil Corp. has already started supplying it’s BS6 compliant fuel to its many stations in the major cities and is planning to provide it across the nation asap until then you can use the regular BS4 fuel in your BS6 compliant engine.
There are many automakers like Mercedes Benz, Kia, MG has already worked on the BSES norms and is started selling their BS6 compliant engines in the market. So if you are planning to buy a new car which is BS6 compliant you can check them.