When a typewriter mechanism is very dirty, with congealed oil, dust, eraser crumbs and general dirt, I've resorted to dunking the whole mechanism in soapy warm water. Using a lot of washing-up liquid and thoroughly drying and oiling afterwards it works, but still felt a bit 'uneasy'. (Also taking care to not get the keys wet, water would get under the glass (or celluloid) and mar the card keytops.)
Stumbling on an article in the February 1957 issue of Popular Science magazine, this dunking in soapy water is actually recommended as a method used by professional repairmen too. With illustrations, using an Underwood portable that would've been ~30 year old by then.