WD40 is a solvent and can dissolve grease. Dont use it around freshly greased bearings.
I use it to flush the crud out of old frewheels (then oiling), and for rustproofing inside frames.

You can get an old clunker going by dripping oil into the main bearings, if you dont want to do a complete stripdown. Any lube is better than none, but oil and grease dont mix well. If you want to use the bike long-term , there is no substitute for a complete overhaul, and rebuild using new balls, and if neccessary, new cones.