From what I've read, 5 is a silicone based brake fluid and has a higher boiling point than 3-4. Because it is silicon and not glycol based (like 3-4) it does not absorb water and reportedly does not ruin paint. People have been running it in older systems for years without problems. The only thing you'd have to worry about is if it eats the seals, which I would think would happen pretty quickly if it were going to.
https://www.thesamba.com/vw/forum/viewtopic.php?t=409242&highlight=dot5
https://www.thesamba.com/vw/forum/viewtopic.php?t=409242&highlight=dot5