An apparatus for continuously preparing viscous compositions containing additives includes: (i) a mixing device for uniformly mixing and dispersing an additive into a viscous fluid; (ii) one or more servo motor driven pumps for feeding the viscous fluid to the mixing device; (iii) one or more servo motor driven pumps for feeding the additive to the mixing device; (iv) means for supplying the viscous fluid to the viscous fluid servo motor driven pumps; (v) means for supplying the additive to the additive servo motor driven pumps; (vi) means for dispensing a viscous composition containing the fluid and the additive from the mixing device into a container; (vii) a programmable logic computer, constructed and arranged to control the operation of the servo motor driven pumps for the viscous fluid and the servo motor driven pumps for the additive, so that a predetermined ratio of RPM between the servo motor driven pumps for the viscous fluid and the servo motor driven pumps for the additive, is maintained irrespective of pressure surges in the supply means for the viscous fluid and supply means for the additive; and (viii) a temperature compensation algorithm for compensating fluctuations in temperature of the viscous fluid.