Had the exact same problem on my very first 428 FE; only came out of one sidepipe exactly 29 seconds after idle at a stoplight. Weird resolution on mine! It would clear out after I took off. Seemed just like what Mr. Fixit said, and then I kept searching and searching... 1) Checked for bad valve-stem seals, No.... 2) Intake gasket and vacuum leak checks, No... 3) Had intake angle surfaced to match heads perfectly, No... 4) Checked for bad/stuck/clogged PCV-valve to carb from valvecover, No... Engine had good compression too... 5) Tore engine apart... and found out the wrong rod bearing clearances & rod bearing side clearances had been set up by previous engine builder.
Oil was slinging up on one side into the cylinders from the bottom side, later the other side too... the
oil rings could not keep away enough
oil at long idles for some reason. Hopefully yours will just be an intake manifold gasket or valve-stem seal as Mr. Fixit suggested. Good Luck.