W124 mate i run on Magnatec . If you go to the Castrol web site and click on oil recomendations it will give you the right oil for your engine. Or any maker of oil they should have the same checking idea.You dont have to have Castrol.
As optimusprime says oil manufacturers websites will give you their recommendations for their oils, along with the quantities.
Here's a link to Mobil where you can check the amount required just by entering your Reg number. http://www.mobil.co.uk/UK-English-LCW/carengineoils_which-oil.aspx