pushit7,
The GM default source should be 100.
Anything in that source will sort to the appropriate source during a price update.
Always put any new GM parts in source 100 when you add them to your database.
If you want any items in a different source, create that source and in PMU set that source not to update that source during a price update.
Also, you can run an update manually at any time.
It doesn't have to be done at month end or new month.