I don't think the molecule size of mineral oil can seep into the rubber of the capacitors. I have heard this from multiple sources (reputable). MOSFETs have a positive temperature coefficient meaning Rds (on-state resistance) increases with heat. Device such as BJTs have negative co-efficients (not that this helps you!).
The only other thing I can comment on is don't submerse any PCI cards that have relays on them (ie mechanical).
I say go for it. I've been curious about this myself.
Oh, and if the oil gets hot enough just throw in some chips
