If you go to the main shop forum page there is a poll about what brand oil members of this forum use. Castrol and Mobil 1 are tops with the people here. NO matter what oil you use make sure it is changed often. If your car is a diesel get diesel rated oil. If you do a search of this forum for "oil" you will find more threads than you will be able to read in a day and enough good adivce and opinions to help you make a good decision.
1995 E420 - retired
1986 420SEL - retired