Not to second-guess the Adobe Support people, but there's one thing you didn't mention that might be important:
When you published your X5 Webhelp to the server, was the server running a separate program called RoboHelp Pro (or RoboHelp Server)?
RoboHelp Pro (it was renamed RoboHelp Server sometime around X5/RH6) is an advanced server software package, with extra features, but it is not necessary to publish Webhelp on a server. If you weren't using the Pro/Server software before, you can go ahead and install RoboHelp 8.
If you aren't sure whether you used RoboHelp Pro/roboHelp Server, you should ask the server administrator.
Yes, we have a server which is running "RoboEngine" and we publish all our help in "WebHelp Pro" format to this server.
Just adding to Elisa's excellent comment. From your "RoboEngine" response I suspect you were using RH X5 (authoring client) to publish WebHelp Pro to the RoboEngine. The old eHelp/Macromedia RoboEngine technology is more than 7 years old. So, it would make sense to upgrade to Adobe RoboHelp Server 8, especially now that you have already upgraded to the more modern RoboHelp 8 authoring client. (I believe the Server upgrade price is $999)
The new Adobe RoboHelp Server 8 has a better search facility, security model, and a much easier installation/administration interface.
A logical workflow would be to back up your old RH X5 project and then open it in RoboHelp 8 which will automatically upgrade your original project to the new version. Then, after the RoboHelp Server 8 is installed, you would publish your project as "WebHelp Pro" to the new server.
Also, when you say "we," does this mean there are other authors on your team? If so, you want to make sure everyone is on the same version (RH 8.)
Adobe Certified RoboHelp and Adobe Captivate Instructor