Actually, there are several, all equally correct, answers, but only one real
one.

The original 15 degree diesels (Golf, Jetta, Dasher, etc.) all had a capacity
with filter of 4.5 liters. The original Vanagon had a capacity of 4.0 liters
with the 603A pan. The later European Vanagon 1.6L TurboDiesels with the
different pan (number ?) had a capacity of 4.5 liters. The official oil level
is then given by the pan number. Old pan ... 4.0 L, new pan ... 4.5 L.

But the optimal oil level can easily be obtained for your installation at
your powertrain slope angle using the tried and true Smokey Yurnick method.
To wit:

1.) Put 3.5 liters in the crankcase after filter change and run the engine
until fully warmed up.

2.) Park on a straight and narrow.

3.) Connect up tachometer to diesel. Use W terminal on Alternator or
equivalent approach to measure idle rpm.

4.) Add oil in 0.2 liter increments until engine rpm drops.

5.) Stop engine, let sit for 15 minutes, check oil level, mark dipstick.

You will have determined the maximum level of the oil sump before the onset
of heavy crank splash and oil aeration. Actually, I've been known to siphon
out 200 mls. then add 100 mls. back.

Alternatively there is the borescope method. Add oil and watch the onset of
frothing.

Frank Grunthaner