I have a spreadsheet I prepared to calculate the different number of miles to break even on the cost of GDE vs. diesel saved and the 40K number is right in the middle. It also depends on how you drive your truck and where you drive it. In Oregon we have lots of hills. In the short time I've owned mine with GDE I've greatly appreciated the engine braking feature of the GDE. Haven't towed yet but sure I'll like that feature even better while towing. Captainmal speaks the truth about emissions and nasty dirty black oil. I'm no chemist but I'd bet nasty dirty black oil is NOT as good for any vehicle as golden brown oil.