I steep the bodies in a lukewarm mix of washing up liquid & water.
If they need polishing I use Autoglym super resin polish.
Some 60s brands of slotcars had a metallic finish, don`t use TCut or other colour restorers on these car.
Some of the Scaley cars made of the soft plastic (eg Panther,Europa Vees) don`t polish up quite as well.
Some folk use floor polish (Johnsons Klear I think) on these to good effect. I never have.