Fluid dynamic bearing, spindle motor, and disk drive device

A fluid dynamic bearing filled with a lubricating oil composition, the composition containing a base oil and an ionic liquid, wherein the ionic liquid is an ionic liquid containing:

• at least one cation selected from the group consisting of a tetraalkylphosphonium cation represented by formula (A) below and a tetraalkylammonium cation represented by formula (B) below, and
• at least one anion selected from the group consisting of a bis(trifluoromethylsulfonyl)imide anion, a tris(trifluoromethylsulfonyl) methide anion, a borate anion represented by formula (C-1) below, a borate anion represented by formula (C-2) below, and a borate anion represented by formula (C-3) below:
• (in formula (A), R1, R2, R3, and R4 each independently represent a linear or branched alkyl group having from 1 to 18 carbons, and
• in formula (B), R5, R6, R7, and R8 each independently represent a linear or branched alkyl group having from 1 to 18 carbons)
• (in formula (C-1), R9 and R11 each independently represent a linear or branched alkyl group having from 1 to 22 carbons, and R10 and R12 each independently represent a hydrogen atom or a linear or branched alkyl group having from 1 to 22 carbons).