Need to replace O-ring at oil pressure switch. What is better Nitrile, hydrogenated nitrile (Hnbr), or Viton O-rings. At first it look like Viton are best but they are not so good at lower temperature and at google it say they work down to 15F or 5F. In MN we go to -25F every winter over some nights.

 

well, if the existing type of orings have lasted you until now, I would say that replacing with similar material orings should last until your car dies of natural causes.

 

^^^ Give up at Vitons. It is questionable for very low temperatures. Will give to someone else to check. Went with Hydrogenated nitrile butadiene rubber (HNBR). It is Nitrile with something over and it is, per Wikipedia new wave in car industry.