I've searched several places but can't find a method for calculating viscosity of a lube oil as a function of temperature. Typically oil viscosity is given by the manufacturer at two points, xx SUS at 100F and 210F or xx cSt at 40C and 100C. How do I calculate the viscosity at my actual operating temperature using the given data points?

Hi Compressor Man,
Many years ago I found the formula using for calculation the oil viscosity for temperatures more than 100C.
Vt = V100 x Ct, where
Vt - kinematic viscosity at required temperature (cSt),
V100 - kinematic viscosity at 100C (cSt),
Ct - averaged coefficient that equal
Ct = 1/[(e raise to t/103) - 1.63]
Maybe it will be useful for you.
Regards,

Check out this Shell Toolbox

Compressor Man, sorry to use your spot.

Mr Hughes thanks for the tip, excellent tool!!

Mineral oils follow behaviour of Newtonian fluids. Oils of the same VI can be plotted on a linear graph. On a XY linear graph you will have to plot the known values of Viscosity at KV 40 and that of KV 100 and draw a straight line between the points. For any temperature between 100 and 40 Deg C just draw a perpendicular and arrive at the KV x. In fact you can extrapolate the line above 100 deg C or below 40 deg C and still have an accurate KV for that temperature.

Hussam Adeni