Weighing is always more precise than a volume measurement.  I would consider weighing the amount (and this is directly proportional to the volume via a constant called density...).

Weighing is indeed a better idea. Then you may consider adding your sample TO the pre-weighed viscous additive.

Another alternative is to make a stock solution with a solvent to allow easier addition with the pipette. Especially useful when you are going to prepare multiple reactions.