Exhaust manifold leak

[JMBT]

   Hey guys here’s what I have got. CM2250, @m*m^2, 645K, oil or soot, or both coming from 2nd cylinder exhaust port leaking past the manifold gasket. Truck is running great, fuel mileage good, power good, but manifold temps are a bit higher than normal. Cylinder cut out test, runs on all cylinders individually, but sounds equally crappy on each, so I am guessing normal for the age of the motor.
Could the oil be from failing valve guide seals? Truck also smokes a bit after I have idled to warm up in the morning. Then quits until cooled off again.
Brother and I replaced the manifold and gaskets 160K ago, but before the truck was demandated. Since deman the exhaust temps have been higher in hard pulls. Could the manifold have shrunk?? I have heard of that happening before, however none of the bolts look bent.