Even though titanium is corrosion resistant, titanium will readily combine with oxygen and nitrogen in certain conditions.
When screwing an aluminium BB cup into a titanium BB shell if the threads are good then grease of any kind will be OK. If the threads are slightly distorted, damaged, out of round etc then there is the possibility of galling.
Simply, galling is where metals start to behave more like velcro than solid metal.
Any aluminium BB cup will have some Al oxide (a hard ceramic, abrasive grinding material) on the surface of the threads and this will scrape the surface of the Ti BB threads. The freshly exposed titanium surface that has been scraped, will now want to react with some thing. That some thing will be what ever it is touching.
The conservation of energy means that the work done in turning even a very slightly tight cup into a BB shell will create heat. This all leads to galling. Titanium is a poor conductor of heat (for a metal) so localised hotspots occur.
Think of the BB cup as a VERY blunt cutting tool when reading the bit below.
http://www.titanium.com/titanium/tech_manual/tech2.cfm
"Working with Titanium: Titanium is highly reactive and will react with its environment at relatively low temperatures. When it is heated in air, a self-protective, titanium-oxide film, which is very adherent, will form on its exposed surfaces. In many corrosive environments, the film becomes a barrier and, in the absence of abrasion will decrease the corrosion rate."
"Titanium has a tendency to gall, and its chips can weld to the cutting edges of the tool. This is particularly so once tool wear begins. Sharp tools should be employed at all times and should be replaced before they dull. The feed should not be stopped while the tool and work piece are in moving contact."
"Titanium has been classified as difficult to machine due to its physical properties. Heat caused by the cutting action does not dissipate quickly because titanium is a poor heat conductor. Titanium has a strong alloying tendency or chemical reactivity with material in the cutting tools which cause galling welding, smearing and rapid destruction of the cutting tool. "
Still thinking of an aluminium BB cup as a very blunt tap?
Good.
By the way, why do people use copper grease on bikes?
Molybdenum disulfide comes in various forms: grease, oils and dry (spray on).
Rocol amongst others do Dry Moly lube, as an aerosol that dries almost like paint.
But for threaded parts Loctite do a range of thread lock and thread seal.
