FWIW,
For FSX SP2, either of the toolsets Doug mentioned in reply #290 don't appear to work correctly in either Blender 3.0, or 3.0.1.
When exporting a textured only, Specular Material Mode selected, MDL it throws an error "[02/03/2022 09:04:42] EXPORT ERROR! Couldn't find the corresponding shader node for material <cube>. It should be a 'Specular BSDF' for specular material.".
I was getting this error with something for FSX I made, but it also throws it as shown for a textured default cube.
The parts were UV unwrapped and the textures made in GIMP.
In case someone else has encountered this problem, this hack has worked for me: I found that with a part I had already made in Blender 3.0.1, if I opened the file in 2.93.7 and saved it and reopened it in 2.93.7, that the textured MDL would then export.
Edit: This hack seems to be OK if you only want to export a completed simple model. If you are going to do any futher work on the object, I have since found that it is better to copy the object from 3.x and paste it into 2.93.x. The warning from Blender when you open the newer file in the older version (expect loss of data) at the minmum refers to some UI items missing, e.g., no snapping option in the UV Editing window header. .
If one needs to use this toolset, it looks like staying Blender 2.93.x will be easier.