Does anyone know if I can use Futaba 3PK crystal (TX) in an Aitronics M8?

No, you can't !

You can try it, there may be some overlap and glitching could occur.

I have heard it both ways. I have seen one company say that the radio makers claim you need to use their crystals because they want to sell you their own brand at heavy markups.
I have read that you should use a matched pair from one manufacture. I have seen it said you should use the brand of the tranmitter. I have receiver which insists it will only work if you use crystals from them.
Does anyone have real information on whether there are real differences.

It wont work, I've already tried it.

27mhz AM and FM are generally interchangeable (for sure on AM) 75 mhz AM & FM are rarely interchangeable. The only swap that I have had successful on 75 AM was KO and Airtronics.

I agree to slot. I've been doing this when I have the same freq on the track 27mhz AM band. In FM band I haven't tried it...

I take it you are saying that on the 27mhz band the different brands are interchangeable; i.e you could use 27 mhz AM Futaba Crystal on a 27mhz Airtronics transmitter or receiver

not that you could use 27 mhz AM crystals on a 27mhz FM transmitter and receiver.

just got an m8 on 27mhz and tried my ko 27mhz crystals in it and it would not work. both are fm