Put it on!!! I had 340,000 on mine when I did it with no problems, I saw no improvement in mileage but i sure enjoy driving it more so don't expect it and be pleasantly surprised if it does improve. Even 25 hp is a noticeable difference so if your worried about it don't get the high hp tunes and be smart about how you drive it.
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Put a Turbo on it!!!! 
'95 F-250 Powerstroke, 350,000 Miles, Mods: Phoenix chip, 140V IDM, Diamond Eye DP to 4" straight pipe, CCV mod, Amsoil synthetics throughout.
'04 F-350, XLT Lariat, Powerstroke, CC, Dually, SS exhaust, AirRaid intake and filter, Edge Evolution, PHP FICM programmer.
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