[Old Truck Man]

could be valve guides and could also be rings. since you state their isn't any smoke on startup that indicates its not valve guides or seals. deceleration going down a long steep grade will create exceptionally high manifold vacuum. that can suck oil past the piston rings & valve guides even if their not worn that much.. If you have excessive puff back (blow by out the oil fill) its a sign that some compression is leaking past the rings. I would suggest pull the valve covers and make shure nothing is plugged. install a PCV system . A PCV don't hurt HP or fuel economy. What is does is revieve crankcase pressure a bit. it removes moisture and combustion gases from the crankcase. it will help keep the engine cleaner. Unless the engine is using excessive oil & fouling plugs I wouldn't worry about smoke after a long deceleration. A PCV system and electronic ignition are two upgrades I will always recommend.