The Unseen servant can apply oils and potions if instructed; however, it cannot activate any magic items that would require a roll (use magic device). As it is mindless, it would most likely be unable to operate something such as a horn of fog.
In the case of the dust of appearance, tossing dust should be considered a simple enough task.
Regarding the potions and oils, it depends how you normally rule someone able to administer a potion or oil to a conscious character.
The rules say that you can personally drink a potion or apply an oil to yourself as a standard action.
It says that you can administer a potion or an oil to an unconscious person as a full round action.
It says nothing about administering a potion or oil to another conscious person. I know many will disagree with me, but I rule that you cannot give someone a potion or apply an oil in the middle of combat. Outside of combat you may feel free to let someone pour something down your throat or apply an oil as the circumstances are different.
There is no rule saying they can and none saying they cannot and I usually rule with common sense in a situation where the rules are silent. After all we cannot (but often do) expect the rules to cover every conceivable circumstance or permutation.
So I would not be inclined to allow them to use potions and oils, but I am sure you or someone could make a favorable argument to sway me the other way.
The same holds true for activating of items. I do not feel the purpose of Unseen Servant should be to skirt the basics of action economy. It is done enough with familiars in my opinion and that makes them targets for me as the DM so they usually do not do it for long as Unseen Servants are easily taken out with a simple area of effect spell.
It makes sense to not be able to give a conscious person a potion- they'd need to at least make themselves flat-footed.
It doesn't have lungs, so I don't think I'd allow blowing the fog horn. It can't speak, so most items won't work without specific text allowing it.
I did completely leave out the entire "use in combat" portion.
When it comes to a potion, though the unseen servant can bring the bottle to the lips of the person needing to drink it. This saves the space of hands or for having to use an action to find the potion (should you have it at in a normal bag). The action of drinking a potion is still a standard action for the person drinking the potion (if unconscious it is a full-round for both because it has to be done carefully). So in this case, the imbiber of the potion is not saving a standard action.
In the case of an oil, things get a bit trickier. This will really depend on what the unseen servant is applying oil; if the person or object in moving, if is is an object is it in someone else's possession, can the servant apply it to the appropriate surface, etc. This would be a more case-by-case scenario best left to the GM in the field.
With both potions and oils in combat, I see either the unseen servant or the potion/oil getting destroyed by an attack of opportunity since it is a really low AC to hit in most cases.
Now I agree with the idea that an Unseen Servant or companion could bring the potion to your lips so to speak in combat, but that you still need to use your standard action to drink. It would also be nice if your hands were full with sword and shield, etc.
This could save move actions to draw or ready a potion or oil so I see an advantage in just that alone.