The oil I use is of neutral fragrance, not greasy, and washes off you and out of your clothes easily. Unless people are very hairy (which is where you need a lot of oil sometimes!), you don't generally need a shower, although some people might prefer that anyway. Where I work there are showers available for the clients' use if they wish, but most don't.
If you have a strong preference for a specific (type of) oil, you can bring your own and ask the therapist to use that.
I leave it up to the client if they want to get undressed completely or not. In my opinion a full-body massage includes the buttocks too, and that is of course easier if the pants are off.
But I respect the client's wishes at all times of course, so always make sure I understand what they mean by full-body (a lot of people actually mean everything apart from the buttocks and the front of the torso!) and they can keep on as many items as they want. I leave the room for them to get undressed and dressed. And I use a drape, uncovering only the areas I'm working on. I place a small towel over the breast when I get to the stomach area. Do you know the trick where you pull a table cloth from underneath the tableware without them even moving? Similar thing: towel over the breasts, then pull sheet away from underneath so they don't get exposed.
For a full body, if you want a bit more than just very gentle (or rushed!) stroking, I would recommend booking 1.5 hours or longer, especially on the first visit where some chatting will usually precede the actual massage. I would also recommend seeing a qualified massage therapist, and not going to a beauty salon, as they are usually relatively expensive, and many (not all) of their therapists have only had a basic training in massage, alongside all the other stuff they have to learn. They often only apply light pressure, which is of no help if you want some knots sorted out in the process.