Often, mechanical oil pumps are specified because many of our competitor's gear drives require the oil pump for lubrication.  This is because they sell multipurpose, "industrial", gears that are adapted for use in cooling towers.  Amarillo Gear Drives are designed specifically for cooling towers and use a non-wear oil slinger system that operates in either direction.  A mechanical oil pump on our fan drive is an extra cost option but is not required (unless an oil filter option, a pressure switch option, or operation below 450 rpm input speed are selected). The oil slinger is installed and functional, even when the oil pump is added as a redundant lubrication system.
Our mechanical oil pump is truly bi-directional.  It will pump in either direction while using the same suction and discharge ports.  Many years ago, we worked with Viking Pump to design and manufacture this bi-directional oil pump. The reason other gear drive suppliers warn against running in the reverse direction is that they use a standard unidirectional oil pump and they do not have the oil slinger as a redundant system.