Weird Valve Seal Question?

Ok, for the last month my truck has been smoking on startup, which you would think is the valve seals but it only does it about 2 times a week. It can sit for the whole weekend and not runs and you start it and it DOESN'T smoke. Then you can let it sit for 4 hours and it smokes on startup. Can it be that just one valve seal is gone and it just depends on where the position of the valve is when the truck is shut off? Or is it something totally different. Any help would be great. This is what I have done since this started (along with other stuff that I found along the way)

Oil change with lucas oil
changed timing belt and adjusted the timing
Took off valve cover and checked of sludge/oil passages blocked/etc, everything is clean with no debris
changed both belts
new exhaust from the manifold back

Smoke color?Have u run a compression check to see if 1 cylinder might be way lower in comp. than the rest?BJ

Its oil thats burning. I haven't done a compression check. If it was blowby or a leaky ring wouldn't it smoke all the time? I've been keeping a close eye on it and even though its cold out if it doesn't smoke on startup, absolutly nothing comes out the exhaust. And now I know its all comming out the pipe because I just did the exhaust.

But good point and now I'm going to check it anyways.

my truck does this too, and it has new valve seals in it, it is white smoke, i thought it might need rings but i am not sure

blue smoke oil...white smoke...coolant....

My point exactly.If the truck smokes blue, ur burning oil,if the smoke is white,ur leaking coolant into the cylinder head from a blown head gasket-something Bseries are notorious for.Ususally,trucks smoke at start up as worn rings or worn valve seals let a small amount of oil into the cylinders when the truck cools off at nite.If u put a new exhaust system on it,if the old one was stopped up,as most are,u may have reduced the back pressure just enogh to allow a little blowby from the valves.If u dont live in a cold climate,next time u change oil try going to the next heavier grade oil and see if that helps. BJ