Base Oil are specified like 100N / 150 N etc. What does this 150 & N specify ? Any idea ? If this 100 / 150 are the viscosities then how higher viscositiy Oil say 460 VG, are manufactured from this low viscosity base oil ?

Here N stands for neutral.Generally base oil describes as N 100 , SN 150 and BS 150.The numbers may not be indicative of vicosities.Because SN 150(solvent neutral) or N 150,whose viscosity @ 40°c is 30-35 cSt.In the same way BS 150,bright stoke 150 has a viscosity @40°c of more than 500 cSt and this oil is being used for making VG 460.

An base oil manufacturing company representative can give correct difinition.

Base Oil are specified like 100N / 150 N etc. What does this 150 & N specify ?

The Base Oil viscosities generally follow the Saybolt system of viscosity classification. The lighter grades N (Neutral) and SN (Solvent Neutral) are measured at 100°F (SUS @100°F) while the heavier grades BS (Bright Stock) are measured at 210°F (SUS @210°F). The numbers represent the Saybolt Universal Seconds (SUS) value at the temperature of interest.