Hi Paul,
I have both 6.5 and FSX but have not tried the two together.. mainly as I struggle enough to get FSX to run with clear skies

It is SOLD as being compatible mate so it must be. The only proviso is that ASGraphics and ActiveRadar do not (as yet) work in FSX so how the hell you're supposed to choose your clouds is beyond me. In all honesty, I've not read the readme's or the AS forum so there may be pointers there
Oh.. make sure you have FSUIPCv4 too
EDIT...
Paul.. here is the section on FSX Compatibility
FSX Compatibility Information
ASv6.5 now officially supports Flight Simulator X using FSUIPC v4.0 or later.
FSUIPC v4.0, or the latest available version can be obtained from
http://www.schiratti.com/dowson.html. To install FSUIPC v4, download it and run the included installation .exe.
Important Notes:
FSX includes various improvements to the weather system including improved smoothing, better cloud lighting and depiction, included vertical air simulation (thermals and updrafts/downdrafts), and better turbulence representation.
The release version of FSX and SimConnect (the new interface to FSX variables that FSUIPC v4 uses) does have a few known issues that can affect weather simulation:
1) Winds Aloft
Wind aloft directions are currently not fully settable by external weather programs including FSUIPC/ASv6.5. Thus, variations in actual wind aloft direction experienced can and will vary from the data. In many cases the difference will only be slight... in others, the difference may be large. An update to SimConnect with an associated fix for this problem has been speculated by Microsoft and we eagerly anticipate its arrival. Note that this problem does not affect wind speeds, only directions.
2) Smoothing
FSX does not support direct ambient control of weather variables including wind direction/speed and visibility. For the most part, FSX has improved its own internal smoothing yet some limitations do exist.
3) Depict FS Haze Layer Option
FSX does not support ambient control of visibility, and thus "Depict FS Haze layer" MUST be enabled using FSX in order to have visibility clear as you gain altitude.
4) Graphical weather flashing at the upper end of the sky depiction
When weather is set via ASv6.5 through FSUIPC v4, FSX may depict a flashing area of sky directly above the aircraft when at higher altitudes and during certain times of the day.
5) Vertical Air Simulation
Because FSX does not support direct ambient vertical wind velocity control, VAS is not compatible. FSX does however include its own new thermal and vertical air simulation effects.
6) Aloft Temperature Layers
Certain problems with aloft temperature layers may be experienced, and, similar to the Winds Aloft issue, we expect this to be solved with the first SimConnect update (not officially announced but speculated and highly anticipated!).
7) ActiveSky Graphics and ActiveRadar NOT Compatible with FSX
ASG's textures are designed for FS9 and are not compatible with FSX. Attempting to submit textures via ASG may result in problematic texture depiction and potential FSX crashing. We are working on new graphics content solutions specifically for FSX!
ActiveRadar is an FS9 module which does not support FSX's new module format. We are working on a new radar gauge solution specifically for FSX!
More information about new graphics, radar and other FSX solutions will be posted on our website at
http://www.hifisim.com as it becomes available.

Potential stutters during weather updating with FSX
If you experience any stutters during weather writes, it is recommended to disable all throttling options in ASv6.5. The new SimConnect interface used by FSUIPC v4 may result in slight stutters for each weather station write on certain systems when throttling is enabled.
9) Using both FS9 and FSX with ASv6.5.
If you have both versions of FS installed and wish to use them both with ASv6.5, you may wish to point the ASv6.ini FS9 folder entry to your FSX path instead of FSX, so that AS can properly update any required station database files. This is generally not required and will not make a significant difference in depiction either way, but may help in certain areas with few reporting stations. If you've pointed the path entry to FSX, there will be no need to point it back to FS9 when using FS9 because such database entries will have already been updated.
10) Potential performance issues when using "Auto Submit" option with FSX
As mentioned, ASG is not compatible with FSX. Using the "Auto Submit" options within ASv6.5 can potentially result in performance issues as ASG attempts to load ASG at each weather update. Thus, it is highly recommended that all Auto Submit options be DISABLED when using FSX.
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ATB
DaveB :tab: