So yes filters are different for everybody in accordance with your RF environment. Here's the method I use. If you do it right it's certainly is not hit or miss. I see filter selection as more of a troubleshooting Aid oh, it doesn't have to be set for every object.
Global filters are not permanent applications, they are used as a temporary sampler. Default for Global filters is normal. Global filters affect every object on your radio unless you go in and manually change that object in site options or Department options...
Filters should be sampled on the radio itself so you see real-time results from RSSI and noise levels and and error rates so make sure your radio is on detailed display.
Sit on a TG in a trunked system. You should be able to see the signal bars, RSSI and noise level and error rate on your display. Go to menu, settings, Global filters and try invert, back out of menu and jot down your RSSI and noise levels while on invert. Note that you've already sampled normal filter as that's the default. After listening to invert for a while go back into menu, settings, Global filters and try wide normal then back out of menu. Look at your results on wide normal for a while, you're looking for an improved RSSI and noise level, noise level is really what you want to focus on. Go back into menu, settings, Global filters and try wide invert. Repeat the process with filters off also. Note your findings and see what filter produce the best RSSI and noise levels. You noticed that we avoided using the auto filters as they sample every filter every time on every object and slow scanning way down and you never know what filter did the trick anyway.
If you find a filter that did better than other filters on that system then go into menu, settings and Global filters and return Global filter to normal. You don't want to use Global filters to accommodate just one system as it will compromise other objects that do well on normal which is default for a reason.
Now go into menu, manage favorites, go to the favorites list that system is in, review edit system, go to that system, edit site, pick a site, preferably you should have the least amount of sites you have to have, set filter, by default it should be on global now you're going to apply the filter that you found was the best choice when you did your sampling. Backup 1 in menu and pick the next site and apply the same filter until all the sites have the filter applied. Back out of menu and you have customized that particular system with the best filter from your global sampling.
Applying conventional filters is a little more tedious as you have to wait for transmissions on that conventional frequency. You will use the same method of sampling the global filters. Just normal, invert, wide normal, wide invert and no filter at all. When you have picked your best filter go back to Global filters and return it to normal which is the default.
Go to menu, manage favorites, pick the favorites list that conventional object is in, review edit system, pick the system that conventional object is in, edit Department, pick the department that conventional object is in, set filter, it will be on global by default, apply the filter you found best during your sampling.
No, you cannot apply a filter to one single Channel. The filter you choose will apply to all the conventional items in that department. You get around that by the way you program your radio. If you have a fire department with five VHF High frequencies, or you are listening to Marine broadcast, or you're listening to Railroad you want to put all of those conventional items in the same Department as they're all going to call for... The same filter.
It is helpful if you add the filter indicator on your display as it will help you to keep track. Because you reset Global filters to normal most objects and systems will have normal indicated. You only went in and made a change because of poor reception or problems with that particular system or Channel and those items will indicate the new filter you applied on the display.
I found this is the most efficient way to apply filters.
PS. I almost forgot, when you're done make sure you hook up to Sentinel and transfer all the changes to your profile so as not to lose them LOL.