We are an Oldsmobile family. We own (2) 2002 Auroras, (1) 2000 Intrique & (3) Aleros. All have 6 cyl engines. 1 Aurora has over 150K, the Intrique over 150K & the other Aurora 83K miles. We get the oil changed every 3K miles, always. All 3 go low on oil between changes, as much as 2 or more quarts that needs to be added. The 83K mile Aurora is diamond white. I see dark or black areas around the tailpipes. We do not know where the lost oil is going. Our mechanic says don't worry about it, just add the oil when necessary. We want to know what possibly could be wrong and where is this missing oil going. It is not leaking out under the car. What should we be checking?