FYI - I have used 20W50, 10w60, 5w30, 15w40, 5w40 (diesel) and even 0w20 in similar mercedes engines, the 60 viscosity oil was useless as it really dropped the fuel economy too, the 5w30 and 0w20 are no good for it for long change interval no matter if the oil was helix ultra (TBN too low)
The best deal was to get the cheapest oil that meets the tech requirements - in this case mobil1 0w40 was and still is the cheapest. Costs me 50 USD for two containers of 4.5 litres each. Why should I spend more money or even similar money on other products when I know its going to waste. lol... if you think that way then go ahead - its your money.
now coming back to the business and knowledge base part. You are recommending oils here on PW based on
1 talking to customers who dont know jack about their cars.
2 feeling oil between your fingers
3 feeling the bonnet for temperature
4 all pakistanis are cheating theives.
till today I have not seen ONE SINGLE POST of yours that spoke sense, yet you advertise your brother's business of lubricant sales. When someone asks you for oil specs you have no answer but to buy LM or kixx and do your (smile) routine - which is comical. 
You may have come from some school of thought that "business means to maintain books" - I dont, I come from a school of thought that says to only do a business if you know what it serves. In your case at least know what an engine is and what dictates the oil requirements and other minerals in the add pack (that would be about 80%). Otherwise you are in the same category as the generic spare parts shopkeepers who sell parts as "parchoon"
Here is a question - if you go to a pharmacist who never graduated from Bpharm, would you trust him?
That is the big Q here.. you can go ahead with your life and promote your brother's business because they are a lot of sheep on PW who still beleive in "feel" - that would increase your sales by several magnitudes.