Here is my take on this subject. Three holes, all different. Engine. Transmission. Primary. Why contaminate one with the other. Each does its own thing. Think of your car or truck, same thing. Engine, trans, diff etc. The engine filter only has to filter out the motors' deposits, not the primary chain and clutches not the gears, just the motor. As far as using one oil when three used to be the way, technology. Why did they have to use 60 weight oil? Technology. Things change. Life gets easier.