Is there anything you absolutely shouldn't use as lube? And does butter work? Fics always seem to suggest that butter could work

If you don't have to worry about condoms possibly degrading, your options are a lot broader.

NOT SOAP. EVER. OH GOD OW. It seems like a good idea at the time, if you're in the shower, but you will SO regret it later.

Same with moisturizing lotion type things. Fine for whacking off.. not so much for anything else.

Nothing with any kind of sugar, glucose, whatever in it. BAD idea. You see this in some massage oils, especially the 'edible' types... this is more of an issue if you're uncut, though it can be bad for your arse as well. Sugar breeds candida infections.

Getting a yeast infection under the foreskin is a hideous experience; happened to my brother, which he'd caught from his GF at the time and was too embarrassed to go to our dad about it... big brother to the rescue. Oiyyy.

Olive oil, any other vegetable based oils can work in a pinch; Crisco is the classic bathhouse cliche.

I personally wouldn't use butter or anything else animal-based, never have, so I couldn't tell you if it'd work. It sounds really awful though.

Have at it.