How is the DHW produced? If it is a heat exchanger in the boiler, it needs to be set warmer. If it is an indirect, it might work, but will cause the system to probably turn on and off more than ideal. The higher you keep the water, the more heat you can get out of it, but you increase the standby losses (and use more oil). Depending on the type of heat you have, 140 might not be warm enough, so there's no easy answer without knowing more details.