Hm, well, I always take tips and essays and such with a grain of salt, because human sexuality is so diverse, people like it and do it in so many different ways. ^^

And I see you're considering lotion or cooking oil, which I think are good substitutes, because they're usually handy (somehow) and they work - using cooking oil can be especially fun to write! :D And ouch, yeah, unless your 'bottoming' character is a real masochist, I'd stay away from stuff like shampoo. >D; I'm in agreement with you that something about it should be included in the writing, though, regardless.

Good luck with your fic!