3D models - dumb or feature-based?

There can be substantial differences between 3D models. With so many 3D CAD applications, 3D libraries are often shared and converted from/to different file formats. During such conversions you will usually loose some "intelligence" of the 3D model.

So you can find many 3D blocks as wireframe only (very limited use), many 3D blocks as meshes (surface facets, no volume data - typically from visualization/rendering software) and many blocks as dumb solids (no features).

The intelligence of models created natively in parametric applications like Inventor or Revit is important when you need to modify/edit the 3D model. Depending on its intelligence, it can be as easy as changing a single numeric parameter or as difficult as redoing the model from scratch. Also the size of the model can depend a lot on the way how it was created. Defining a geometry and features using the native functionality of a parametric modeler leads to small, fast and effective models. Importing the same geometry as dumb objects makes larger, slower and clumsier models.

So make sure your 3D DWG block is really a solid object (not just facets), make sure that your IPT or RFA blocks are really created in Inventor or Revit, not just imported from AutoCAD DWG, IGES or other general (dumb) formats.

What CAD file format is the right one?

From the user's point of view, the right format is the one you use in your CAD. And as the most used CAD application in the world is AutoCAD - and as most other CAD applications can read DWG - the right format for you is probably DWG. The textual version of Autodesk's DWG file format is DXF - and this well documented format is readable for virtually any CAD software.

For 3D CAD data there is no single prevailing format. Of course, DWG often carries 3D data; in mechanical industry, the most widely used 3D software is Inventor with its IPT/IAM format, there are libraries (entourage, gaming models, cars...) based on 3D Studio old 3DS format or new 3ds max format MAX. But the safest way for sharing 3D data is one of the neutral data exchange formats - STEP or IGES. For BIM data in AEC industry you can find many sources of RFA (Revit family) block libraries.

If you have found a block in a different format than that of your CAD application, you can still use some conversion tool to convert the CAD data to the requested format. The safest way in this case is to use an import (or conversion) function directly in your CAD application, or to use the original CAD application to re-save the data to a neutral format (e.g. Revit to open a RFA file and save it to DWG). There are also standalone general CAD conversion tools and web services but the results of such conversions are not always perfect.

An incompatible version of DWG file

One of the most frequently asked questions when using a Block library with DWG files is how to open a newer-version DWG file format in my old version of AutoCAD?

The AutoCAD file format - DWG - is updated in every 3rd release of AutoCAD. So published DWG blocks saved in a newer version might not be readable in an old AutoCAD version. E.g. AutoCAD 2006 cannot open drawings from AutoCAD 2009.

But the solution is simple, you don't need to upgrade your AutoCAD immediately - just install the free tool - Autodesk DWG TrueView (2008 or higher), don't get confused by its name (it can really convert, not only view) and use its conversion functionality to save back the complaining DWG block to any older DWG format. There is no other option for this - various "DWG conversion" utilities do not save the genuine DWG format and the resulting DWG files can cause many problems in AutoCAD.

See the tips:
How to convert an AutoCAD DWG drawing to an older file format?
Error message: File was created by an incompatible version of AutoCAD
and
www.autodesk.com/dwgtrueview-download