Thick Exhaust @ Idle

My wheels: '95 4-door 4x4. 260K miles.

Just got an oil change the other week. Normal oil, not high mileage. If I'm at idle for more than 5 minutes my exhaust gets thick, dark and smoky. The gauges dont indicate that there is a problem. In the past when Ive gone in reverse I've expereinced the same problem but never before at idle.

Any suggestions/thoughts/ideas???

Thanks.

Welcome to the Tahoe forum. 
 
Valve seals, PCV system, rings, cat conv.  Just some ideas.

I have a 95 2dr 4x4 and at about 260,000. is where I changed out my motor for a replacement. You can nurse it along for awhile longer with oil addatives and the likes but it's time to give it a rest. The motor that is.