Most of us feel that there is no approx formula for calculating Viscosity of mixture of liquids.
Now here is a useful co-relation. Thanks to Milt, It is his effort which can be very useful for many of us.
(1) VBI = 14.534 × ln[ln(v + 0.8)] + 10.975
where v is the viscosity in centistokes and ln is the natural logarithm (Loge).
The second step involves using this blending equation:
(2) VBI-blend = [wA × VBIA] + [wB × VBIB] + ... + [wX × VBIX]
where w is the weight fraction (i.e., % ÷ 100) of each component of the blend. In using the above blending equation, it is necessary that all viscosities are determined at the same temperature, for example, 100 °C.
The third and final step is to determine the viscosity of the blend by using the invert of equation (1):
(3) v = (ee(VBI - 10.975) ÷ 14.534) − 0.8
where VBI is the Viscosity Blending Index of the blend and e is the transcendental number 2.71828, also known as Euler's number.