The engine will run on 87 octane. The knock sensor system will make sure the engine does not suffer from pre-ignition. However, you will experience power loss. Do the right thing, use the grade of gasoline that the Volvo engineers designed the engine to run on. The fuel economy will probably be slightly better than with 87 octane. The 5 speed should be pretty economical, especially for long highway trips. You are right about rare - I have a manual trans. V70 in Mississippi, and I have never seen another one anywhere in the South.