There are many base oils for the oils we use every day in our life. Vegetable, petroleum, PAO, silicone, phosphate esters and others. In the shop we use oils for lubes, as hydraulic fluid and to treat metal among other uses.
Lube oils. Most common are petroleum based oil. Once refined the base oil has about a 10-20 ISO viscosity. Since in many cases we want thicker oil a viscosity enhancer is added. The viscosity enhancers are mostly polymers and work well, but do have a life. In highly churned systems, the polymers tend to shear and the oil gets thinner with time. Then there are anti-wear additives and extreme pressure additives. The extreme wear additive is essential in worm gear applications and in heavy load applications like spur gears an bevel gears. The anti-wear package is good for more lightly loaded applications.
Hydraulic oils. These have the functions of a lubricating oil as well as the transmission of pressure. Since these oils tend to be used over a long time they also have anti-oxidation additives and often emulsivity and anti-foam additives.The anti-oxidation additive combines with water in the system and traps same to prevent reaction with the sulfur naturally present in petroleum and prevents acid build up. This additive has a finite amount of water it will capture and then acid buildup starts to occur. Ever smell dark hydraulic oil with a "Burnt" smell? that is the acid after the anti-oxidation additive has been consumed.Burnt oil is pat end of life, replace it quick!
Brake fluids. These are traditionally a PAO base oil and are used in automotive brake systems and some small hydraulic systems. Some newer spec brake oils are silicone oil.
Automatic transmission oils. These oils have all the additives of hydraulic oils as well as an extreme pressure additive and all of the additives are in larger amounts as these are now expected to run the life of the car.
Hydraulic jack oil. Most are simply hydraulic oil, but in those 10,000Psi jacks and porta-powers a very different oil is used. Hydraulic oil made from standard petro base becomes as thick as peanut butter at 10,000psi. You see petroleum becomes more viscous as pressure increases. One exception is Kerosene which has very little viscosity increase with pressure. Most 10,000psi jack oils are very highly refined kerosene based.
Seal compatibility. The seals in our hydraulic systems have to survive a tuff environment. Use the wrong fluid and the seal will shrink and harden or swell and get soft. Has a lot to do with the aniline point of the oil but I digress. In most standard hydraulic systems petroleum based oils are used and in most applications a Nitrile (called Buna N after the German plant that developed it) seal is used. In higher temp apps Viton may be used. Now in a system using phosphate ester oils(Often spec'ed for fire resistance) The Buna N seals will turn to soft sticky mush and be totally destroyed. Add Marvel mystery oil into Buna N seals and same result.
(Most of the Patented Medicine oils sold to Free up or un-stick stuff is hell on regular seals.)
So... have a blower gear box and a hand crank drill press and you need to drip oil on both? Which oil to choose? I would go with ATF! Yes that old red standby Automatic transmission oil. Its cheap and easy to find, safe for regular seals. lubricates all those gears and has a great additive package. It also has a very low change in viscosity due to temp change, so if your shop is 20F you will still be able to crank those antiques.
Written by Jeff Reinhardt