When it comes to bearing surfaces in a gun grease works much better than oil. On the hinge area of O/U's use Krieghoff grease or STOS(Ponsness Warren makes it) Doesn't get runny or hard in extreme weather. I have used STOs on my K-80 guns for years and the current one is just starting to get free enough to open all the way by itself when you open the the lever...but it has had a couple hundred thousand shells through it or so
A lot of gun stores have STOS ..or Krieghoff grease. I just like the STOS because it is clear