I don't think I would use it but I would plan on changing the first batch of synthetic a little early on an older engine since it probably will disolve any crud that may be in there...... I doubt there will be much if you've been using a C rated oil and changing it regularly. When I changed to Amsoil in my truck I didn't use the pre-treatment stuff, just changed the oil.

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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