The Italians initiated multiple "modernization" (for lack of a better word) programs in Libya, and Italian planners regarded Libya in particular in high regard for future Italian expansion (the whole "fourth shore" thing). The Italians built modern highways as well as other projects and, once oil is discovered, this kind of investment would continue to flow in. If Libya remains under Italian control until the late 60s or 70s it could end up with a plurality Italian population as well, which opens up many interesting possibilities come the 90s, 00s, and 2010s.
This likely won't be the case for Ethiopia. On the one hand Ethiopia would be lacking in significant mineral/petroleum deposits to warrant continued Italian investment like in Libya, and on another Ethiopia's larger population (who have a history of independence and have developed a national identity) and more rugged terrain would lead to problems in Italian expansion into the hinterlands. I don't think we'd see a Vietnam level conflict in the colony only because I don't see Britain or France arming rebels there for fear of the conflict spilling over into their own colonies. The US won't disturb a potential anti-Communist ally depending on how the war goes. Maybe the Soviets could arm the rebels, but again, depending on how the war goes, maybe they're too weak to really bother with Ethiopia (besides, I doubt the Ethiopian rebels would really be Communists so much as anti-fascists). I could see Mussolini trying to hang on for the rest of the 40s and maybe 50s, but once he dies the Italians probably start pulling out, de-colonizing the country sometime by the late 60s or 70s.