If we take your post literally he is full of S***. The fuel does not get "inside" the engine (meaning under the valve cover). The fuel stays inside the fuel lines and the injection pump.

Maybe he was talking about the clear fuel filter?

If this person is talking about underneath the valve cover AND around the cam area, that is caused by not changing the oil often enough AND perhaps using the wrong kind of oil (cheap).

You'll need to be a bit more precise before we jump all over this person.

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84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)