I recommend sae 30 older engines since it promotes sealing unless you are in a colder clime & must have a multi. Since it is an 87 I would step it up from 40. If you can afford, you might even go synthetic to increase longevity.
The 3.2 engine used in year 1984-1989 uses 10.4 Quarts of oil.
911 3.2 1988
$300.00
Porsche recommends using Mobile 1 oil, specifically 0W-40.
The 2003 911 C4S takes about 9L of Ow40.
A 2001 Porsche 911 2 Carrera requires 9 quarts/8.5 liters of oil without a oil filter change. Including an oil filter change it will require 9.3 quarts/8.7 liters.
Mobil 1
Porsche recommends Mobil 1 Synthetic 0w-40.
A 1979 Porsche 911 SC has an oil capacity of 8 quarts if completely drained. This is about 7.57 liters.
Penzoil
Take it to a certified Porsche dealer and let the experts service your precision crafted engine.
Porsche recommends the use of Mobil 1 synthetic 0-40 weight.