1. Lumion 6.5 and newer:

We recommend Windows 10.

2. Lumion 6.3 and older:

If you're using Lumion 6.3 or older and need more than 4 GB memory for your scenes, we recommend that you run Lumion on Windows 7 (64-bit with at least SP1).

The reason is that Windows 8 and 10 do not allow older DirectX 9 applications (such as Lumion 6.3 and older) to use more than 4 GB graphics card memory for a given application - and no system memory either. On Windows 7, graphics card memory and system memory are not limited by the 4 GB cap when running older DirectX 9 applications such as Lumion 1.0-6.3.

Lumion 6.5 and newer use DirectX 11 which is not affected by this cap and can use more than 4 GB memory on Windows 7, 8 and 10.