Which video format can I copy and play on an older Video iPod?

The video can't be "HD" (720p or higher). 720p resolution is typically 1280x720. The video playback specs for your iPod model, which is a 5th gen iPod, is shown here

http://support.apple.com/kb/SP41

where it says in Video Support

"H.264 video: up to 768 Kbps, 320 x 240, 30 frames per sec., Baseline Profile up to Level 1.3 with AAC-LC up to 160 Kbps, 48 Khz, stereo audio in .m4v, .mp4 and .mov file formats

MPEG-4 video: up to 2.5 mbps, 480 x 480, 30 frames per sec., Simple Profile with AAC-LC up to 160 Kbps, 48 Khz, stereo audio in .m4v, .mp4 and .mov file formats"

Also, for a 5th gen iPod (the first iPod that could play videos), if I recall, it was released before the iTunes Store started selling video content, such as TV shows and movies. That content has copy protection (DRM) and may not play on your iPod, even if it within the resolution limit. Videos from the iTunes Store without DRM, such as iTunes U and podcasts, may play (within the resolution limit).

There is a command in the iTunes menu bar (under Advanced) for Create iPod or iPhone Version. However, that command is disabled for videos with DRM.

You can also add videos from other sources to iTunes, and they will play on the iPod (within the resolution limit).

"DRM" is digital rights management. For some video content from the iTunes Store, DRM is used to link a video file to an iTunes account, so that only the purchaser of the video can play the video file. This applies mostly to TV shows and movies. Other video content, such as podcasts and iTunes U, should not have any DRM.

NOTE: Songs purchased from the iTunes Store no longer use DRM.

What is "videos with DRM"?