Ahhh Keithl my old chap! How are you? and hello from the wrong side of the pond.
First as has been stated at altitude shutting down an engine is not good. More drag less thrust most likely you will have to descend. I suggest requesting higher from ATC(if you still have some way to go) or flying at the correct LRC from your AFM or AOM. Now if we are talking low level then for best range or endurance at some stage 3 is better than four, then 2 better than 3. Theoretically 1 will be better than 2 also but I wouldn't risk it considering all the other implications. Why? at range speed (@1.3 vmd) at low level your 4 engines will be operating well below their optimum band. Hence shutting one down and allowing the others to operate at a slightly higher rpm will, below a critical weight, reduce your overall gross fuel consumption (kg/hr) and thus increase your specific air range(nm/kg). The same applies at endurance speed(@1.1vmd) where Vmd is best Cl/Cd.
As for the speys leaking oil when wind milling. Yes they did but not much. It was a partial loss system at the best of times and the max endurance with an AAR probe fitted is limited eventually by a. Engine oil or b. Crew sanity.