[1Bad62Pro/Street]

I don't know much about the BBV6's or the gas mileages and never known anyone around my woods have one.
On these engines they claim Oil and fuel consumption is low.
I would check for leaks first. Then go from there.
Check the links below:

Spark Plug Gaps:
If you are running the very desirable GM HEI or Petronix system, you need to increase the spark plug gap. The reason is the HEI and the other one produces more voltage and amps. I would recommend starting out at .045 for the HEI and .040 for the other one.
You will notice a much smoother idle after ditching the points. Take it out and run it through the RPM range. Lug the engine some and then wind it out to max RPM's. Does every thing feel and sound ok? Increase the gap by .005" until the idle starts to suffer and drop back .005". After that is all done, install another new set of plugs two heat ranges colder and try expanding the gap once again. It is unlikely the plugs will carbon foul which indicates either a rich mixture (too large of jets) or the plug range is too cold to burn off deposits. The idea is to run the widest gap possible without misfires and a smooth idle. The wider the gap, the smoother the idle quality and better high RPM power will be. To top that off, milegae will increase, too. Those air/fuel adjustment screws on the front of the car is for the idle ciruit only. Those screws do not lean out or richen the mixture at all. What they do is increase or decrease the amount of air and fuel equally to the idle circuit. To adjust: turn screws in or out to get the best and highest idle, then back in to get a lean drop of 20 RPM's. A 50 RPM drop is ideal, but your carb may not like it. This does nothing to hurt fuel ecomony or performance, but does clean up idle emissions.

http://www.6066gmcguy.org/performance.htm
http://www.6066gmcguy.org/gmcv6a.htm
http://www.6066gmcguy.org/gmcv6b.htm
http://www.6066gmcguy.org/GMCplaid.htm
http://www.6066gmcguy.org/351v6.htm
http://www.6066gmcguy.org/401v6.htm
http://www.6066gmcguy.org/478v6.htm