5x is possible. If you set the "throttle" at 1800 rpm with the transmission out of gear, the only loads on it are friction and pumping losses. It will not use much fuel. The same engine loaded to max torque will use a lot of fuel.
Remember, diesels aren't like gas engines, there is no throttle. There is a lever we call the throttle which chooses the set point of the rpm governor. The engine's injection system then tries to add fuel until the set point is reached. You don't control the fuel, only the rpm - the injection system controls the fuel. Once the lever is set, the injection system will attempt to maintain rpm regardless of load.
It would be harder to reach 5x on a gas engine. It has a real throttle and consequently large pumping losses at high rpm and low throttle settings, requiring more minimum fuel to keep it going. On a gas engine as the load is varied the throttle must be adjusted to maintain the desired rpm, as you are controlling the fuel delivery with the throttle.