I have a bottle of degreaser and used it on the derailleur. I was planning to replace the chain (stretched), cassette (broken teeth) and maybe front sprockets (worn) but they are riveted to the crank arms so maybe not those. Bottom bracket is a Shimano sealed cartridge bearing one so that can stay (previous owner upgrade I guess; it has v-brakes instead of the original cantilever). Undecided about using a new chain and cassette with worn front sprockets or temporizing by cleaning the old cassette and chain.

Apart from that, tyres are too worn, front wheel rims are so pitted I couldn't smooth them with sandpaper and they don't brake well, bar grips are worn and torn, plastic pedals worn smooth - lots of replacements. And changing the handlebars for the fun of it. Lots of paint chips and dings on the fame being filled with car touchup paint which looks awful close up - but I had it and the colour match is close, so it's cheap and tidier. https://spray.bike/ is tempting ... but not this bike.

Inspirations: https://www.youtube.com/@bkefrmr - Bike Farmer, who is trying to be the 'Bob Ross' of tidying up 90s steel bikes and https://www.youtube.com/watch?v=C4n2S7-SC4Q - Spindatt "So I restored the original paint on my vintage MTB" and his followup vid on the process.

Mineral spirits for the grease. Paper towels. Brake cleaner or starting fluid if you want to get it completely oil free for paint or whatnot.