1. Any silicon-based oil lubricant is ideal for plastic parts. A common product is "tire shine" or (preferably) Aerospace 303. Even Armorall will work.
2. Your biggest problem will be over-lubricating. Exposed oil -- exposed to outside air, even under a keycap -- tends to collect dust (which is mostly shedded human skin particles)
3. MX Cherry keycaps are friction-fit, so ideally you want the oil to work down inside the switch itself, but clean it off the top "X" part so your keycaps stay nicely on.
4. As long as you're going to the bother, this is an excellent time to install O-rings.