I realize a 5 speed is a nice upgrade to our cars....but I'm not considering one.
I just drove my 4 speed 72 Bavaria over 2,000 miles. Bellingham, WA to Monterey and back...at least one day over 600 miles.
I got right at 20 MPG for the trip. Maybe I could have eked out 22 with the 5 speed?
The engine sound would be less with the 5 speed, but that sound is more like music to me than noise. The wind and road noise would be exactly the same.
I used 1/2 quart of oil on that trip. I might have used less with an OD?
Our cars ran at 90MPH all day on the autobahn "back in the day" with a 4 speed.
Even if there were measurable fuel economy and comfort considerations with an OD, the trip to Monterey was probably a one time adventure for me (and a great one at that!

). I just generally don't do enough hours of highway driving at a stretch to rationalize the benefit of an OD against the cost and the work.
That's my story...and I'm sticking to it.