Can I use oil or grease to mount the seals?

You can use a moderate amount of compatible lubricant to help the axial movement of the rotating part on the shaft (it must not damage elastomers). For the assembly of the stationary part, it is advisable to use only denatured alcohol, that lubricates at the time of assembly and then evaporates without leaving residue. It is also advisable to mount seals with perfectly CLEAN faces.