The function of the oil seal is to stop whatever fluid is inside from leaking out the clearance between the shaft and housing. The seal may also be used to prevent outside materials, such as dirt, from moving in through the clearance.  

ADVANTAGES 

Good temperature resistance.
Commonly chosen as high temperature replacement for Nitrile or Polyacrylate  

Polymerised acrylic acidesters or PA and ACM.
Good resistance to mineral oil, hypoid gear oil E.P. additive, greases, aging and flex cracking.  

Commonly referred to as Buna-N or NBR and is copolymer of Butadiene and Acryonitrile.
Good resistance to petroleum oils, water, silicone oil, greases, glycol base fluids.
Good abrasion resistance, cold flow, tear resistance. 

Broad temperature range.

Good ozone resistance

Resistant to compression set.