What should I look for in a suntan lotion for a nude camping trip? I don't know what to look for when it goes on the whole body. Are there any ingredients that I should watch out for?
Your mileage may vary, so know thyself.
Start conservative, it is easier to back-off on protection than it is to treat sunburns. SPF 50 is not too high and you may want an even higher SPF when starting. Sunscreen that contains titanium dioxide or zinc oxide is great because they protect against both UVA and UVB. Since they are also inorganic compounds they don't breakdown with exposure to sunlight like other sunscreens, so you might not have to reapply as often (always plan to reapply anyway).
The problem with titanium dioxide and zinc oxide is that both compounds are white and will make your skin look paler than usual. I prefer pasty white to lobster red on any day. I have not heard of someone having a true zinc or titanium allergy but I guess that should also be mentioned (know thyself).
Sunscreen that is OK for your face should be fine for the areola, and you should apply there as well. The best way to manage mucosa is to avoid exposure and that can be handled by staying standing or being careful about positioning when sunbathing.
A skin is a skin, so there's no difference if you take off your T-Shirt or your pants first time in the season. The less the body part is used to the sun, the more protection it needs. Well, the only difference is the skin on the penis, which is usually naturally a bit darker, so it already has more sun protection (which doesn't mean you can't get sunburns there).
For the first whole-day trip I'd take 10 for the parts that were already exposed to sun in the season, and 30 for the rest. After getting used to the sun I'd leave 10 for the parts most exposed to the sun, such as shoulders.
When hiking, I never use sun-lotion to my forearms and calves, except the very beginning of the season. I sunbath occasionally, but from my experience the most endangered parts are breast, shoulders and the back of the knees. If I get sunburns, it's usually there.
I can't help but think that this question may trolling, but here goes anyway.
There is a difference between skin and skin if one includes the glans as it is mucous membrane: anything marked "for external use only" might cause unexpected irritation. Don's humorous remark about capsaicin is a good example that I'm sure more than a few men are accidentally familiar with. Since the mouth and lips are also lined with mucous membrane a lip balm with sunscreen would probably be a safer choice for that particular application.