A a demo user, is it against the rules for me to create and upload skins for the demo cars?

You can make any skins you like, the CMX viewer can be used to preview skins for cars not in the demo version.

Uploading and sharing on the forum is not a problem, to upload to LFSWorld however, you would need to buy some skin upload slots.

Usually, just ask a friend with a s1/s2/s3 license or a random person, send them your skin, and they upload it cuz lfs community nice.

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Thanks guys