I would think it is plausible that a thin oil could get in between the layers of a motherboard by capillary action, at the edges or where holes are, and then embrittle the plastic, leading to separation of layers and all kinds of reliability issues.I have read an article about a company called Green Revolution Cooling who use a special FORMULATION of so-called "mineral oil" called GreenDef. This was the SUBJECT of a year-long test by Intel. Servers are submerged in racks FILLED with GreenDef coolant. Circulation of the fluid dissipates server heat, which is carried out to a centralized pump module where the heat is exchanged to the outside. Also Puget Systems have been marketing oil-immersion kits for years, since around 2007. Puget use a white mineral oil from STE Oil called "Crystal Plus 70T". They say that certain items such as CPU and GPU need special cooling blocks in place of the heatsink blocks used in a conventional air cooled system, that fans are not needed but can be left running because it makes the oil swirl in a pretty way, that wicking effects mean that cables have to be arranged correctly, that spinning platter hard drives should not be immersed in the oil.

A Slashdot article from 2008 had mixed comments, including "I know that air cooling has its limitations, but there is something nice about knowing your computer is not going to spooge all over your desk or floor."