Take a look at the "Automotive Lubricant Engineering Forum" thread on Hydro-Treated Oil.
Basically hydrotreatment is used to improve the quality (i.e. refine) of distillates (i.e. kerosine, gas oil [diesel], and lube oils) obtained from distilling crude oil. One of the most important functions of hydrotreating is to remove sulphur from the distillate. The sulphur compounds are acidic and cause corrosion. Also, when fuel containg sulphur is burnt, it pollutes the atmosphere with sulphur dioxide which causes "acid rain".
On hydrotreatment, the sulphur combines with hydrogen to form hydrogen sulphide which can then be stripped off the sour (i.e. containing sulphurous compounds) distillate to produce sweet (i.e. sulphur free, or virtually sulphur free) distillate.
Most of the distillates used to produce lube oils will have been hydrotreated nowadays.
