but where is AL okay and where is it not okay?  Do the
engine manufacturers have any published comments on this?

Matt, Et Al,

 

Any pressurized fuel or pressurized oil fitting on an engine or engine mounted component should be steel since the consequences of a failure are catastrophic.  Any other fitting CAN be steel, depending on your anal orientation with respect to weight balanced by your affinity for risk. 

 

Let's see, the only aluminum fittings on my "engine" are: upper cylinder oil return lines (terminated in rubber tubing anyhow), manifold pressure tap (with restrictor), crankcase breather, "quick" oil drain (nothing is attached to it in flight) and hmmmmmmmm............... nothing else that I can think of....