Layer height  

RE: Layer height

My understanding: .15 is better for fine detail. I use it for lithophanes. I have not used anything finer for fear of blockages...

My observation: .20 gives a better surface finish using a .40 nozzle.

RE: Layer height

Conventional wisdom is that higher layer heights yield better part strength. CNC Kitchen did a video showing that 0.15mm actually yielded optimal strength ... though Stefan only tested with a 0.40mm nozzle and didn't share any thought on layer height versus extrusion widths. While these seem to be contradictory results, I don't think they are. It's all about adhesion. The more of an extrusion line that is in contact with the underlying extrusions, the stronger the part. The lower the layer height compared to extrusion width, the more squish you have providing good contact with underlying layers. The higher the layer height, the more rounded the extrusion (less squish) and poorer the adhesion. So taller heights can be stronger (fewer layers to worry about) provided the extrusion width is increased proportionally. Lower layer heights are almost always a safe bet as they increase squish. 

• Temperature matters. Hotter temps (added nozzle heat or reduced cooling) help extrusions bond.
• Nozzle size matters. A "magic" layer height with a 0.40mm nozzle won't be the same as with a 0.60, 0.80 or 1.00mm nozzle.
• Print alignment matters. Parts will tend to split if stresses pull perpendicular to the print direction (pulling layers apart). Bending in the same direction may show more strength. The same part printed with a different orientation may be many times stronger. 
• Testing methodology matters. "Strength" can mean different things. Are you measuring impact, tensile, compression or other methods?
• Filament matters. What works with one may be completely different with another of the same type and description.

Stefan has done some other casual testing on CNC Kitchen with his elaborate test jig. Other things he's noted that increased strength (mostly impact and tensile) are:

• Filling a part printed with sparse infill with epoxy.
• Smoothing external surfaces of FFF printed parts.
• Using a filament oiler (though he cautions against this for the questionable results and potential mess on a Prusa surface).

As a result, I keep to the standards of:

• Layer heights lower than 80% of nozzle size.
• Extrusion widths lower than 120% of nozzle size (and definitely not exceeding the width of the nozzle surface area).
• As hot as possible for strength.
• Nozzle as big as possible to reduce perimeter wall count and increase layer height.
• Part print orientation to reduce stress along printed lines.
• Filament selection based on intended use. Polycarbonate and nylon are going to give far different results at the same settings than PLA or PETG. Carbon filled filament may be many times stiffer, but prone to snapping at lower points.

I don't have nearly the testing capabilities of Stefan. I'm limited to twisting parts and hitting them with hammers, and subjecting parts to tensile strength testing using a carefully calibrated English bulldog. A part that is strong may shatter more easily when whacked with a hammer, or pulled from a different angle, so be sure of what you really want to test.