The amperage changes by the cube of the change in speed or impeller diameter. If you double the speed of a pump you will need eight times the amperage. The increased amperage can be caused by a change in the product. The motor was sized for a low specific gravity fluid, but the lines are being flushed or tested with water. The specific gravity of the fluid has increased for some reason.
Some viscosities increase with a lower temperature, some with a higher temperature.
The viscosity of a liquid can increase with agitation. That is how cream becomes butter. The increased amperage is caused by two part rubbing together as a result of shaft displacement. A bent shaft. The rotating assembly is not dynamically balanced.