The gentlest is basic sewing machine oil and swirly motions with cotton wool. If it's very dirty you might have to try different things. I have used microfiber cloths well wringed up in soapy water. A resin based car polish will lift up further dirt. Don't use any thing too harsh, the black shellac finish will ruin if exposed to alcohol, and spray can oils are known to have ruined decals. Be careful. Once clean and polish a gentle wipe down is enough to keep it clean and shiny. I polish my 201 once a year with Autoglym extra gloss sealant (what I can easily buy here, there are lots of similar stuff). The "sealant" or "glaze" version is the same as the polish with out the gritty part, but you might need the gritty part during the initial cleaning.
You can safely use spray can stuff on the internal parts if they are very grimy; if only a bit dry a good clean out of dust and sewing machine oil will do the job just as well, just takes a bit longer too loosen old grime. Metal bits like needle and presser bar will clean up like new with the right polish, as well as the chromed plates. If there's rust and pitting it will not look like new again, but it will still clean and polish up well enough.