I am looking at getting a 2010-2014 Chevrolet Suburban. We have taken a handful to our mechanic to check them out. First thing he does after opening the hood is to check the oil level. On each occasion, he has noted that the oil dipstick is discolored, with a blackish tint, and tells us not to purchase the SUV. We have visited several dealer lots and have checked this on each of them. Only one, of about 15, didn't have the discoloration. The one that didn't had a substantial amount of rust on the back end, and ruled out. Any idea what causes the discoloration and if it really is indicating a problem?