Your engine should take about 8 quarts, the info is in your owner's manual also though. Be sure and change the filter also when you change the oil.
Also, since you have an 86 300E, M103 motor, I would consider using a higher weight oil (2nd number) , like 15w 40 , or 10w 40 , or 20W 50 , depending on your climate. I say this because higher weight oil burns less. And your motor probably consumes oil.
2006 E350 w/ 155k miles (Daily Driver)
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)