what is the best oil to use as my car has a leak that needs to be fixed on 1991 Mercedes-Benz 300SE

hi, i bought a 1991 mercedes and i love it.. the person i purchased it from works for Mercedes.. He said if i ever need a part to call him and he'll get it at cost for me...lately i noticed the oil light comes on now and then.. and we think it has an oil leak..what i'd like to know is would you call the person i bought the car from? or would you take it for an oil change somewhere else?? thank you,KD