1988 GMC Van oil usage

Twice while driving along @ 65-70 mph without accelerating there was a huge puff of black smoke that came out, It indicates too rich of a mixture could be caused by the following:

Possible causes include a bad O2 sensor, excessive fuel pressure (bad fuel pressure regulator or plugged return line), leaky fuel injectors, dirty air filter or restricted air inlet, or a defective coolant sensor that prevents the engine management system from going into closed loop mode.

I just purchased the van and drove it 500 miles. It used about 2 quarts of oil on the trip and the fuel economy was terrible.

No oil leaks and 2qts of oil for 500 miles is no good-get the engine compression checked could be valves/rings and cylinder wear

IS IT THE RINGS OR THE VALVES?

If compression is low in one or more cylinders, you can isolate the problem to the valves or rings by squirting a little 30 weight motor oil into the cylinder through the spark plug hole and repeating the compression test. The oil temporarily seals the rings.

If the compression readings are higher the second time around, it means the rings and/or cylinder is worn. No change in the compression readings would tell you the cylinder has a bad valve.