I have 2 routers in an 802.11s configuration (meshed over 2.4). The matching 5Ghz wireless APs on both are set to use channel 40.
When I look at the wireless radio however, it is actually running on 36.
It's not just a display issue - I checked a connected PC and it confirmed channel 36.
Problem is several neighbors are on 36 and 44 but nobody is on 40.
This is normal and expected. Each channel occupies 20 MHz. Since you have 80 MHz set, 4 adjacent channels will be used: 36, 40, 44, 48, as shown below:
The only way for an AP to start on channel 40 is if it is set to 20 MHz width. With 80 MHz width set, the available starting channels will only be 36, 52, 100, 116, 132, and 149 (depending on local regulations).
So if I select 40 with 80Mhz width, I'm likely in contention with the 36's and 44's in the neighborhood?
If so, and there are considerably more 44's than 36's, am I better off selecting channel 36 with 40Mhz width?
Probably not? Interference depends on many things including how many other neighbor AP's are sharing the channel range you are using, whether they overlap (worse) or are using the same range as your AP (better than overlap) and especially on how close in power they are to your AP. For example, if your neighbor's signals are weaker than yours by ~20 dBm, you won't need to worry so much about interference.
The best option is to test your WiFi performance trying different options using the iperf3 package and see how it works. You are likely to benefit more from 80 Mz channel width than you lose out from neighbor interference, but the only way to know for sure it to test it out and see.
Also, the graphic elbertmai posted is a bit out of date for the US. Specifically, the FCC allows up to 1W from 36-48 and from 149-161; and up to 24 dBm from 52-144. Unfortunately, OpenWrt may not allow txpower this high even if your device was certified for it on factory OEM firmware by the FCC. It can be a bit hit or miss, but the point is that UNII-1 and UNII-3 can provide you more range in some cases - especially download throughput.
To confuse matters further, you may find you get better performance dropping transmit power back a bit, say from 30 dBm to 27 dBm, or 27 dBm to 24 dBm. This can be beneficial if it improves your signal to noise ratio.
