I used to see milkshake syndrome with my '99 E320 when it wasn't being taken on long drives. Committed W210 folks will tell you it's "normal", which is the mindset of the 210 set. I'm sure it reflects poor design. The problem is caused by condensation mixing with oil, due to the cap not allowing much air circulation. Mercedes settled a class action early in W210 production which addressed engine sludge by offering a free oil change. That fixed the symptom once, but the problem was never fully addressed. Clean out as much as you can, change the oil, go for a good day trip.