Can I add a run capacitor to a capacitor start motor to increase its efficiency? I remember doing this 20 years ago and the run current dropped 20% or so, but I didn't measure the power consumed. The motor was quieter and cooler. The run cap value was 0.1 x start cap value, connected across the start cap and start switch. This is an offgrid water pump application.
That would be worth investigating. You could try it again and make measurements this time. If you do it, you would be risking overheating the start winding. If you have information that the motor can be reversed by switching the capacitor from one winding to the other, you can be sure that the two windings are identical. If to windings are not designed to be interchangeable, the start winding has smaller wire and will likely overheat fairly quickly if connected continuously with the start value of capacitor. It may overheat eventually even with a X 0.1 capacitor.
If the motor is reversible in the manner described, there will probably be a diagram for that on the nameplate or on a label on the inside of the wiring box cover. You could also check the instruction sheet if you have it or can find it on the manufacturer's web site.