I was always taught if it rides on a rail, grease it. If it rotates on a pin, oil it.
Probably not by a tribologist....
Just remember, grease picks up dirt, debris and other stuff and keeps it trapped, oil, being lighter and runny tend to wash out crud when you reapply it.
A lubricant that always stays in place is a double edged sword. The same goes for a lubricant that tends to be more mobile, it tends to not stay where you want it.
This is why wheel bearings are greased, and rotating engine parts are oiled. Wheel bearings are sealed against dirt ingress and do not produce much in the way of wear particles. Engine parts tend to produce more wear particles that need to be flushed out to the bearing areas and trapped in a filter.
Making blanket statements about which type lubricant to use is actually bad information, it always depends on many things.
-How many wear particles are expected to be generated between lubrication renewal?
-How likely is external contamination settling/getting trapped in the lubricant?
-What kind of external contaminants are expected to be seen?
-What is the temperature range expected?
-What are the expected normal forces between the moving parts?
-How sensitive is the system to drag caused by thicker lubricants?
-What kind of sliding speeds are expected?
And just a few more things, just because you put some lubricant on a frame rail doesn't mean it is stays there lubricating forever and ever . . .
Grease works as a lubricant by holding oil in a matrix, usually some form of soap. Think of the matrix as a sponge and it has soaked up some oil, under no-stress conditions, the sponge holds oil, but when placed in between to sliding surfaces under pressure, the oil is squeezed out and performs exactly the same as regular non-grease oil. Once the oil is squeezed out of the sponge much of it runs off, and some is reabsorbed by the matrix, but given time and cycling, the oil in the sponge reservoir will become depleted, and all you will be left with is the matrix, which looks exactly like it did before all the oil was squished out, but is not much of a lubricant.
Also, any lubricant, grease or oil, can be rubbed off a surface. Grease or oil the slide rails of your favorite pistol and work the slide back and forth a few times. Now, separate the frame from the slide and look at the rails, much of the lubrication has been pushed off the actual contact areas and had built up in the non-contact areas. In such cases, a thinner lubricant would be preferable, as it can flow back into the areas vacated during cycling.
Lubrication is unfortunately not something that cannot be summed up in a short pithy sentence.