It's not a whole lot, but i noticed that when the car is backedup into the garage and i rev it.. it stains the wall with some black stuff, might be carbon or oil. Bad head gaskets?
If there's no blue smoke in the air it's probably just by-products of combustion. It's not uncommon for the exhaust to leave a blast pattern on a wall.
Oil can get there from the rings, valves, or pcv. If the motor runs well otherwise just check the intake manifold for oil. If the intake is wet with oil, you may have a pcv system issue.
probally just a very rich condition. no smoke= hopefully no trouble. pull codes from the computer. not a head gasket problem. when cold the 5.0 will tend to run rich. that is when it is in the garage. does it do that when warm?