Check the valves and GP's first and then go from there, starter fluid isn't going to do any harm unless you use it stupidly (use too much, make the engine race with it etc.).

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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