their tanks were made by Opal (a GM subsidiary)
I came here to check that, but you seem to be right - see the wikipedia article on Opel (note correct spelling)
http://en.wikipedia.org/wiki/Opel The company ... has been a wholly owned subsidiary of General Motors Company since 1929.
Makes one wonder about the ways we are helping our enemies today
You don't have to wonder - google "US arms sales" or "us arms sales to dictators".