First off I'll remind you that everyone's skin is different, will tolerate some things better than others, is predisposed to certain issues. So there's no one universal answer to skin care questions. There'll be a few good "rules of thumb" but beyond that you have to try things and see what works best for you.
Powders are good for keeping skin dry and promoting ventilation so your skin can breathe. This can help keep away problems with yeaste and other funk that grow in moist conditions, reduce sweating, and prevent itching. They also help prevent skin irritation from rubbing, as they kind of offer a dry (graphite-like) lubrication. Most powders lose their effectiveness if they get wet. You can choose from corn starch, talc, and other medicated varieties like zeesorb and monkeybutt. Cornstarch is usually considered nontoxic, but talc can be harmful if inhailed and some studies also link it with ovarian cancer. Talc dries more than corn starch, and can overdry skin and lead to painful eczema. I use talc mainly on my inner thighs where my diaper rubs when I walk, and also under the side wings of my diaper where my hips would otherwise have problems breathing and start to sweat, itch, and grow pimples. Powders inside your diaper will lose all effectiveness if that part of your diaper gets wet, directly or by wicking. My skin has a strong fungal biome so I use zeesorb AF on my feet and crotch to keep the funk in check.
Cremes serve many of the same purposes as powders. Some work as barriers, leaving behind a greasy film that prevents other contaminants from reaching and irritating the skin. Most are water-repellant. Many will react unfavorably with adhesives in diapers, most often causing elastic bands to separate from leak guards and leg gathers. (as an example, DC Amore diapers are horribly affected by walmart's baby lotion) Allergies to lotions are much more common than reactions to powders, so make sure you try them in smaller quantities at first, to avoid a possibly painful large rash. It's easy to transfer lotions to the front panel of your diaper, where the oily film from your hands will coat the panel and prevent tapes from sticking, so you have to handle it carefully when diapering yourself. You may need to wash your hands after applying lotion before you handle your diaper. I don't use lotions much.
Baby lotions work a bit like oils in that they can somewhat form a barrier, and are extremely good at fouling tapes, but their main function is to trap moisture in the skin and prevent drying. They're very runny compared to lotions so they can be more difficult to apply with any precision. Oil is a great way to treat eczema. I don't use oil much either. You may run into problems if cremes get "into orifices", but baby lotions seem to be tolerated very well if they happen to creep a bit into say, your urethra. (lotions can burn!)
So take that information and use it to deal with any problems you're experiencing. Sides of your hips sweating and itching under the plastic wings? Apply powder liberally after diapering. Friction problems between your cheeks or thighs? Apply powder (lthighs) or lotion (cheeks). Butt getting a rash from exposure? Apply a creme or oil before you tape up. Do what you need to. Don't do what you don't need to do.
