Depends on the gun. Sig P series need grease on the rails, and I also grease the barrel lug and the bearing parts on the slide. I use SlideGlide for that. The action parts get a drop or two of either Hoppes oil, or CLP (Weapon Shield or Breakfree) The outsides get a wipe down with CLP. The Poly guns (Glocks and HK) get mostly CLP, although I do put a tinny dab of grease on the slide rails.
Rem Oil is a great product, but I find it evaporates rather quickly. I had a shot gun that developed a rust spot, even though I used to take it out and spry it with RemOil every two or three months. I might have let it go for 6 months when that rust showed up. My storage guns are now coated with Rig grease. I do use RemOil in the action of my bolt gun for winter time. It lubs really well, and doesn't gum up in cold weather.
I think I'm going to switch to grease on the rails of my Sig. For whatever reason, I've tried lots of different oils and they just don't seem to stick around very long on my Sigs I've owned. Normally I use Break Free CLP for most other stuff or sometimes Hoppes #9.