Stuff like that usually won't hurt anything and some of them actually do "work" to some extent. I wouldn't be adding anything with teflon or stuff like that however. I would be more inclined to use something like Lucas or PowerService before MMO but that's my preference.

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k