I personally don't recall it but we did play Bulgaria at the end of the 1984-85 season after winning the old 2nd Division and just before Jim Smith left.
It's mentioned in the book 'A Complete Record' by Andy & Roger Howland:
"After the season ended, United went to Bulgaria to play their national team at the invitation of the country's president. Bulgaria had recently beaten East Germany 1-0 and Platini's French team 2-0 in World Cup matches and United were to provide warm-up opposition before the Bulgarians' vital qualifying match against Yugoslavia. Before 30,000 fans, Oxford were beaten 5-0 by a side which went on to defeat Yugoslavia and qualify for the World Cup Finals on equal points with France."