No question about it: valve guide oil seals.

That is the test for V/G oil seals; run the car to 4000rpm in second gear and release the gas and coast to 2000rpm and jab the gas. Actually this test used to work well but doesn't on cars with catalytic convertors. The convertor usually eats the smoke. We used to test for this this way before cats.

The reason for the smoke is that under decel the intake man vacuum goes major high which sucks the oil by the seals and the engine is not running (fuel is cut off on decel). When the gas is applied again the mixture is filled with a mess of oil.