Document Type in OBA7- Restriction to Use a Z or Y ID's

Hi Guys,

I have never been clear on whether the User defined Document Types in OBA7 is impacted by the conventional rule to always use either a Y or Z for customised objects in SAP. Does any one have any experience as to if thee is any risk of user defined document types cleaned upafter upgrade etc.

i like to use a "K" ID for a new document type I intend to create but want to be sure I am not exposed to any risk.

Thanks,

Ade