Rudy, (jumping in here for Bruce)
the differential should only be filled with the rear axles straight, not drooping, as if the car were sitting on the rear wheels. If the axles are drooping, diff oil runs down into the axle tubes, resulting in overfilling.
I use a pair of ramps to raise the rear of the car for this, since the weight of the car stays on the wheels and the axles stay straight.
Not sure if the entire car should be level, I just raise the rear of the car. My ramps are only 5-6 inches high, so this probably doesn't make much difference.
George Davis
'69 280 SL Euro manual