The topic of the Variable speed drive is an interesting topic as so much publicity is being made.  Energy saving is the slogan.

Yes, when selecting an Oil-flooded air-compressor, you should in most cases select a VSD !

Why ?  

Because the power absorbed at unload can reach very high levels like 80% of total power at 50% of max flow.   The reason why this is so high, is that the Oil-seperator in the Oil-flooded compressor is like a bottle of Champagne.  You shake it and open the bottle.  When the cork is released, a fountain of tasteful Champagne is wasted.  Useful in the formula 1 celebrations but less useful if the oil leaves the Oil-seperator with dramatic consequences.

 To reduce the huge energy loss in partial load, a VSD is needed for an Oil-flooded air-compressor.

The better solution is to invest in an Oil-free compressor with low losses at partial load, like Centrifugals and Industrial Recips.  Better energy efficiency and less waste produced