Low fluid causes cavitation in the pump and the torque converter which is what makes the whining. If its low enough that the pump can't make enough pressure then the clutch packs will slip causing 'incorrect ratio'.
Oil leaks are usually the output flanges. I wouldn't expect a Mackies-fitted sump to be leaking, but if its been 'serviced' since it left Mackies its anyone's guess what state the sump is in. Mine was stuck on with bathroom sealant when I got it and most of the oil escaped on to the A303 on the way home
The gearbox on the black FE on which I've been testing the manual swap was lost to some muppet not fitting an o-ring on the oil filter
