Have you changed the oil (and oil filter, if you have one)?
Was there water in/with the oil when you drained it out?
How fast were you driving on that 30 mile trip?
How long were you travelling over 40 mph?
The road draft tube is ineffective under round 40 mph. Go for a 30 mile trip at 50+ mph.
Better yet, drive 30 miles out and 30 miles back (at 50+ mph).
