Contrary to popular belief, wd 40 really isn't a starting aid, it will act as a fuel and allow the engine to fire just like diesel fuel however. Wd 40 still requires glow plugs to fire most of the time. Either will start the engine no matter what, but it must be used with care or damage can occur.
If you need to use either to start a 6.5/6.5, 1) disable the glow plugs so that the either doesn't ignite spontaneously. 2) It is best to have a helper spray a small shot of either into the air cleaner inlet port while you crank the starter, using the least amount of either necessary to start the engine. Either can be used in this manner if all glow plugs have failed and there is an urgent need to get her running. A small shot of either can almost be a necessity if the temps are super cold, the oil is thick, cranking speeds are slow etc, just remember use as little as possible.
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