So as the topic suggests, I wonder, what are the deciding factors for when to use oil and when to use grease on firearms for lubrication (not rust prevention).
Discuss…
Grease on slow moving parts and small amount of oil on fast moving/spring loaded parts for me.
Bolt rails, lugs: grease.
Bolt and trigger: oil; very sparingly.
Ooo this is going to ruffle some feathers 
i put grease on high wear areas like bolt lugs ( on actions with lugs at the rear ) and the cocking arms of my shotguns and oil along anything metal to metal that has to slide but is not under a whole lot of pressure like a rifle bolt. Being in the autmotive industry i have access to alot of greases and oils to try but guns are also different to cars
A light mineral oil, very sparingly used on everything, auto pistol slides, bolts, revolver cylinder pins, shotgun hinge pins, everything, including my Swiss bolts. Cheers
Interesting… So why would one use oil in some areas and grease in others. What’s the science and reasoning behind it?
None, it is personal preference based on upbringing
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Too much oil or grease inside the bolt, around the spring and firing pin can create a seal that acts like a hydraulic ram and hinder free movement causing light primer strikes. Seen it happen a few times on new rifles where people have neglected to strip grease from bolt internals before firing. Can take a while for it to ‘clog up’ so might not happen immediately.
Thats my reasoning, anyway.
Plenty of science in machine lubrication, just like why you don’t put grease in an engine and why theres different specs for both oil and grease
Yep As @AusTac says and @Gwion Grease for the more friction load wearing surfaces like the bolt heads. Although I have never seen such a tight one that would cause the gauling that is talked about. Then the light oil for the slide points. I like the Silicon oil very slippery does not attract dirt like the oils. But it needs to be applied regularly. Inside the bolt. This depends on you and how hard your bolt is to disassemble. But a light lithium gun grease sparingly to as not to hinder quick action. If it is easy and you like to play silicon oil in there too works.
I would agree for most mechanical applications, as a former Fitter & Turner I have a good understanding of lubrication needs for various purposes from machine slideways to crane hoist drum clutches to high speed rollers etc, but I maintain that grease on a firearm is problematic. The US Army forbids it on all weaponry, Colt, Ruger, Smith & Wesson & Remington trained armourers never use it, so why would I? Also, the “wearing in” of auto pistol slides does not create galling and any wear seen can usually be found to be caused by grit carried in excess lubrication. But to each their own, I only use light oil for everything, Cheers.
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I’m the same.
Firearms are simple machines. Any light oil and most greases are OK in my book.