Good idea to eliminate all that stuff first. I just did my valves for the first time (first time I did it anyway). They were all just a little tight. Seems to be a bit smoother but not that big a difference. Maybe the mileage wil improve too. Anyway, try that stuff first, it sure won't hurt and doesn't cost that much if you do it yourself.

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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