I change shocks based on handling and the appearance of oil. I have an 88 300 SEL which needed new shocks at about 80,000 mi. It tended to nose into the curves and felt unsteady. I checked the front and rear shocks and found oil leaking from both. I replaced these with German non MB orininal part units (BOGE) for half the price and 28,000mi later I'am stil happy.