The near-universal maintenance item on CLK55s is oil seepage. Valve cover gaskets are the most common leak point and are frequently done preventively (roughly $800-$1,000), while a slow rear main seal weep is described as extremely common and not urgent, and oil filter housing gaskets also leak with age.
M113 oil leaks: valve covers, rear main seal, oil filter housing
Mercedes-Benz CLK55/CLK63 AMG (C209) · mechanical