My pillow cover is very dirty, contains lot of hair oil which over the time pilled from my head. When I wash it with my clothes washing soap, leather do not produce from soap and a scum like material is formed. How to wash pillow cover efficiently?
If the pillow slip is or contains cotton, it may be impossible to completely remove the skin oil, which is a fat secretion. One of the chemical characteristics of all fats (plant or animal) is that they bind to cellulose (which is the main component of cotton) and are almost impossible to remove. Treatment with a strong hydroxide (lye, for instance) will convert the fat to a soap which can be washed out, but will also generally destroy the cotton.
Degreasers, such as dish soap, trisodium phosphate, etc. won't debond the fat from cotton, thought they will effectively remove any that's not bonded. That likely won't help much, however. The only real solution is to replace the item, then wash the new frequently, before the oil has a chance to bond to the cotton.