An apparatus is disclosed for monitoring characteristics of a fluid lubricant contained within a lubricant chamber formed in a wheel hub mounted for rotation about a stationary wheel axle. The apparatus includes an annular bearing spacer configured for mounting on the wheel axle between axially opposed inboard and outboard bearings, the annular bearing spacer forming an interior lubricant reservoir between an inner surface of the spacer and the wheel axle, a guide vane for directing lubricant from the lubricant chamber to the interior lubricant reservoir during rotation of the wheel hub assembly about the wheel axle, and a sensor operatively associated with the interior lubricant reservoir for determining a condition of the lubricant.