Hi Eyal,
When you start LAYISO command, you can specify "Settings" option before selecting object on the layer you want to isolate, then select "Lock and fade option", then fine-tune fade value, then select layer you want to isolate.
Here how this looks in the Command window:
Command: LAYISO
Current setting: Hide layers, Viewports=Vpfreeze
Select objects on the layer(s) to be isolated or [Settings]:S
Enter setting for layers not isolated [Off/Lock and fade] <Off>: L
Enter fade value (0-90) <50>:
Select objects on the layer(s) to be isolated or [Settings]:1 found
Select objects on the layer(s) to be isolated or [Settings]:
Layer 0 has been isolated.
Command:
HTH