In this work we present an in-line fiber-optic viscometer for internal combustion engine lubricant oils. The sensing probe is a single-mode single-mode (SM-SM) fiber configuration formed by splicing two sections of uncoated SM fibers with a lateral offset. A rounded-end fiber tip is created to configure a compact in-fiber integrated Michelson interferometer. The measure of the viscosity is carried out by monitoring the shift of the interference pattern.