Usually, low viscosity mineral oil is used. But you have to ensure that your sample can not be mixed with mineral.

Another way is to use geometries with a solvent trap. But be sure to do motor adjustment if you do it like that.

Good luck

Dr Wang above is correct and I would just add that a low viscosity mineral oil can be used in oscillation creep or relaxation testing (if compatible with the sample), but not usually in viscometry, where it can mix into the perimeter of the sample giving erroneous results.  In this case a solvent trap is preferable if available.

The goal of a solvent trap is to raise the local atmosphere around the sample to near saturation with the most volatile solvent in the sample so that it cannot dry out.  It can be useful to spray a little solvent (or water if water based) onto the inside of the cover and shake off loose drips before enclosing the sample to help it saturate quickly.  Also be  sure to put the same solvent in the reservoir to make a vapor barrier.  I attach an application note on using our solvent trap system - most others operate on the same principle whether the geometry has a reservoir built into it or it's separate, as shown in the attached document. 

Wishing you every success with your research!

Girija. Philip is correct and you can use silicone oil and recently i found people are using vaseline too to prevent dehydration.