In SX Microcontrollers, SX/B Compiler and SX-Key Tool, g_daubach wrote:

The various OSX?? settings from LP1 up to HS3 control the setting of the three FOSC2...FOSC0 bits in the Fuse word, where settings from 000b up to 110b select one of the seven available gains for the internal oscillator feedback driver that outputs to the OSC2 pin (111b selects an external RC network).

There is no general rule which OSC setting to use because it depends on the type of the external resonator/xtal connected to OSC1/OSC2. Some components come with internal capacitors (like the "blue" Murata resonators available from Parallax), others require external capacitors, parallel resistors, etc.

As a rule of thumb, you can say, the higher the clock frequency, the higher the gain setting should be. But as Bean mentioned, sometimes, it requires a bit of experimenting to find the right setting for a particular resonator/xtal. It is correct: Higher gains result in more supply current, and also cause some more EMI.

Finding the lowest setting that works for a clock device is fine, but you should consider the possible ambient temperature range. When you minimize the gain, it can happen that the oscillator will not start at lower temperatures.

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