Heavy Smoke, at Times

[raymr]

My car normally has a clean smoke-free exhaust, except for some times in the morning while the engine is still cool. One route to work goes down a short steep hill with a stop at the bottom. When I accelerate from there, heavy clouds of blackish grey oil smoke pollute the air for a few hundred yards. Then it clears up. If I take another route that avoids the hill, there is no smoke at all.

What mechanically causes this? A combination of worn valve seals and a downward angle? Oil pump issues?