If you use a heavy duty 30 weight oil with detergent can it break loose carbon deposits in the engine and possibly affect the EGR and other related systems causing the engine to run rich?

The other question is this: Do the O2 sensor and EGR valves work at start up or are they only engaged after the car reaches a certain temp? The black smoke blows out the tail pipe immediately and continues even when warmed up...trying to rule out the O2 and EGR so that I can possibly point to the cat sine it is already tuned up and I have replaced the coolant temp sensor already. It's a TBI car by the way.

Thanks.

 

The oil is not an issue.
Egr does not come on until the car warms up & I believe O2 sensor is not active until the engine goes to closed loop, again engine warmed up. I would suspect something in the fuel system, like injector not sealing, stuck pressure regulator.