yeah, I have a PC in my workshop, and it's in a sealed chamber, water cooled.
The motherboard sits inverted, so if any water leaks from the cooling system it drips away from the board. Not the most efficient method since the coolant nearest the CPU is highest, and so gets the hottest coolant next to it. The inlet and outlet for the graphics card is at the "top" of the card, so the bottom in the orientation that the machine is in for the same reason. It's certainly not impossible for this to cause problems in the event of a leak, but it's less likely at least.
I'm using automotive antifreeze/water mix as a coolant, since I have lots of it, and the workshop can get cold in winter (It's in the garage.) The radiator is a heater core from one of my old cars before I sent it to the crusher, with the car's heater fan blowing through it. (It's robust, tolerant of a hostile environment, moves a lot of air, and does so fairly quietly.)
In any event, I would not use oil.
Oh, for a refrigerant, you ADD oil to it for the compressor, but it should only be a very small component of the refrigerant. The leaks are most likely to occur on the "hot side" in any event, which would be well away from the circuit board components (So, between the compressor and the expansion valve) I have the components from an old refrigerator/freezer which I've been thinking of making use of in that regard, but need to ensure that I have compatable blocks and lines (Aluminium most likely) before I can make use of it. The worst things about this are that they're heavy, need inflexible hosing, so the PC won't be easy to move, and the possibility of condensation on the cold side putting moisture where I don't want it. Probably all this adds up to an unuseable system, but I'll probably try it someday anyhow.