I was toying with the 5th gear upgrade, basically swapping to what would be a 6th gear in place of the current 5th gear. But since half of my driving is at that 35 mph I would be willing to be I couldn't go that slow, even at idle in the upgrades 5th gear. Lug, right now if I idle (903 rpm) in 5th I go about 28 mph on the speedo and about 25 in real life (GPS). Maybe I should do a run home at idle for kicks
But how does the engine hold out` Are they designed to withstand such low revs and still go for 300,000 miles? If an engine dies too soon it is a much bigger strain on resources than the few drops of fuel it saves..
I can't say from personal experience, I haven't had one last that long. My first TDI was a write off after an accident at 151k miles. I later sold the driveline to a friend that wasted his (don't use veg oil in a TDI). My second one I bought with 175k and sold at 295k. Still running as of last correspondence with the present owner. My third I bought with 150k and had that one written off as a total loss at 220k+ miles. That driveline is in my present car, now with 250+ on the engine.
I do know that the newer (1999+) TDI have variable inlet geometry on the turbocharger and aren't as forgiving on long duration, low rpm and low boost pressure operation as my first generation TDI with their fixed geometry turbos.
I also wouldn't think it would be an issue lowering the 5th gear as long as you aren't idling around in 5th all the time. Since you can't lug a diesel the only think I would worry about is the turbo not being exercised or maybe the turbo running all the time at a lower RPM? Either way at 55+ it is just like having a 6th gear.