All of a sudden 302 runs rough

Replaced plugs, wires, rotor cap points and condenser. Points set at .017. The car started always started immediately every time for about a week after all these parts replaced. I even took it out on the highway to "clean it out". I bought some oil additive today and added it to the engine and started it per the instructions to let it idle for 5 minutes. I left it running, but when I came back, it had died. Numerous attempted restarts and it sounds like the distributor in 180 out, or there's a massive vacuum leak. It started normally for the 5 minute idle, but now it barely runs.  Checked the point gap numerous times and it's .017, swapped out the old condenser, no help. I tweaked the distributor, no help ( it's back in the original position, now) any ideas? I never revved the motor, so I don't think the timing chain skipped, although, when I increase the rpms, it smooths out a little, but dies when it goes back to idle. Maybe a carb problem, but I never touched the mixture screw ( 2 bbl).

Did you check your vacuum hoses for a leak?  Block off each one and see if it makes a difference.  Other vacuum leak sources are the carburetor base gasket, the vacuum advance unit (though usually doesn't cause the symptoms you mention), and the power brake booster.  Also, make sure the vacuum hose didn't come off the transmission vacuum modulator.  These are all cheap (free) and easy to check. 

Good luck and post up what you find.

If you suspect a vacuum leak, you can do the carb cleaner check.  Spray that around the intake manifold and vacuum hoses while its running and see if the idle changes.  If needed, you can turn your idle screw up a bit to get it running smooth while you try this.

Grab a timing light with a tach on it and a vacuum gauge.  Check your timing and adjust your idle mix and adjust them for the most vacuum.

Hard to say for sure what went wrong if you weren't around when it died to notice any weird smoke or sounds.

If it was running well before the additive and poorly after then I would lean towards the additive being the culprit.  Making adjustments will only compound the problem. I am a electronics technician by trade and was told long ago if it was working yesterday and not today an adjustment probably isn’t the problem.  The distributor didn’t move itself, nor anything on the carb. This problem occurred only after adding the additive correct? Could be something as simple as fouled plugs…
 

What additive did you use? 

I used a

RISLONE (sp?) oil additive