"How do you change the viscosity of base oil to make finished lubricants?"
To better answer this question, let's first clear up a few terms and then take a look at the refining process.
According to the ASTM D6074-08 Standard Guide for Characterizing Hydrocarbon Base Oils, a base oil is a base stock or a blend of two or more finished base stocks to produce finished lubricants, usually in combination with additives.
A base stock is a hydrocarbon lubricant component other than an additive that is produced by a single manufacturer to the same specifications (independent of feed source or manufacturer's location), and that is identified by a unique formula number or product identification number, or both. So technically you wouldn't change the viscosity of a base oil to make a finished lubricant.
Viscosity is a measure of a fluid's resistance to flow. It describes the internal friction of a moving fluid. A fluid with high viscosity resists motion because its molecular makeup gives it a lot of internal friction. A fluid with low viscosity flows easily because its molecular makeup results in very little friction when it is in motion.
In the distillation process, crude oil is heated to a vaporous state. There is a temperature difference in the fractioning column. Near the bottom (closer to the heat source), the temperature is higher and becomes progressively lower toward the top. As vapor rises in the fractioning column, the temperature drops. The vapor is then condensed and collected in various trays at differing heights in the column. The higher boiling vapors condense first in the lower trays, while the lighter fractions such as natural gas and naphtha or gasoline are near the top.
In their book, Lubrication Fundamentals, D.M. Pirro and A.A. Wessol describe the boiling points at atmospheric pressure as follows:
The distillation leftovers in the atmospheric column are generally called residuum. Because this residuum has a tendency to decompose at approximately 700 degrees F (371 degrees C), it is sent to a vacuum distillation tower for further processing. The vacuum allows the residuum to boil at a much lower temperature, but the process is the same. As stated previously, the side cuts are pulled off the column at different heights, with the heaver fractions toward the bottom at the higher temperatures. From the vacuum distillation tower, these fractions are sent to a “stripping tower” where steam is used to strip out the remaining lower boiling materials, in effect fine-tuning the viscosity of the different side cuts and adjusting the flash point of the fraction.
The oil molecules are grouped by size and type with typically five to seven fractions. Lubricant base stocks are in the range of 26 to 40 carbon atoms. The molecular weight of each fraction corresponds to the fraction's viscosity. Once the various viscosity fluids are drawn off, they are further processed to remove the impurities and refine the oil.
After the completion of the refining process, you will have base oils of various viscosities. Depending on the refining process used, the finished base stocks will have different performance characteristics. At this point, it is important to note that the oils will not necessarily fall within the specifications of the ISO viscosity grades.
To get base oils with specific ISO viscosities, these different fractions or base stocks are mixed together in different ratios. The addition of various additives will have an impact on the fluid's final viscosity. Some additives such as pour-point depressants and viscosity index improvers are added specifically for the purpose of adjusting the viscosity of the finished lubricant at specific temperatures.