Personally, I wouldn't choose anything with any moly content for roller or ball bearings. My own choice for steering head bearings would be LM lithium based high temp/pressure grease. If 'creep' and subsequent lack of lubrication is anything of a concern, build a relube into your service schedule - it's not like it's a massive job to do.
Everything I've read / been taught has indicated the following:
Molybdenum based lubricants are designed for sliding contact bearings (plain bearings, splined shafts and the like) - the molybdenum leaves a deposit on the surface which acts as a 'dry' lubricant, not relying on any other movement to distribute it.
In a roller/ball bearing the moly builds up on the contact surfaces, decreasing the very fine but essential clearance between ball/roller and race. In a low speed bearing (such as a steering bearing) you'll likely get away with it for quite a while before the deposits cause the bearing to feel 'notchy' - whether this will happen before the bearing is otherwise worn out I don't know...