So, if I don't remove cylinder, then how do you lube the cylinder rod? Also, how to lube internals on centennial without removing side plate?
Cylinder rod, AKA extractor rod -
I agree the answer, generally, is don't lube it, hardly at all. Open the cylinder like you're reloading. Push on the muzzle end of the extractor rod as if you're extracting spent shells. Now the rod is exposed on the rear of the cylinder. CLEAN that area under the star and on the rear face of the cylinder. Little bits of powder, bug poo and dirt tend to find their way there. If it's oily, those bits tend to stay there. I think the rod needs a tiny bit of lubrication as it's a pretty tight fit. I use my bare fingers after handling a bottle of gun oil or after lubing some other part of the gun. If I have a little oil "weeping" out of the side plate or onto the trigger or hammer, I clean up that oil with fingers, patch, Q-tip or whatever, then use THAT tiny bit of oil to lube the extractor rod. A drop of oil on that rod is too much, IMO.
There are two styles of extractor rods. One has a square channel milled into the rod to align it with the cylinder. The other, newer, has a flat spot milled onto the side of the rod. The older, square notch, style is probably more positive in it's alignment, but it's tolerances are tighter and so may require a bit more attention to lubrication than the newer D shaped rods.
IF you operate the extractor rod with the cylinder and the yoke removed, it is not well supported. It will tend to bind and feel rough and sticky. It may cause you to think "I need a lot of lube in there". But put the thing back together and it will all be aligned and move a lot smoother. It's not the oil doing the smoothing, it's the alignment.
Side plates and internals -
There's really only three "bearing" points under the side plate that may require lubrication. One is the trigger, it has a pin through it that rides in a bearing boss. Two is the hammer, it has the same kind of pin and boss bearing surfaces. Three is the rebound slide, behind the trigger. The Rebound Slide is a comparitively large chunk of metal that rides on the relatively rough surface of the inside of the gun.
All three of those bearing points could be lubed without removing the side plate by simply spraying or dripping a very small amount of lube into any of the openings around the hammer or trigger. Then dry fire the thing in several positions (firing position, inverted, barrel up/down etc) to help move the oil around. Oil will seep out onto the trigger and hammer as you do that. Wipe it off, and as suggested above, use that excess oil to lube elsewhere on the gun.
I think that no matter how little oil you put on a revolver, it'll actually be too much. Some will weep, some will splatter when you fire it. If you live in the desert as we do, I think that's more problematic than if we lived in Antarctica or Portland or the Florida Keys. Haboobs will drive abrasive dust into everything and the sand will stick to any oily surface. If we lived in California it wouldn't be a problem. They've passed a law making dust storms illegal. And another law probably makes gun oil illegal so it's not a problem over there.
Sgt Lumpy