The best cleaner I've found for the engine is spray silicone lubricant. Use the cheapest stuff you can find. (That may be what WD-40 is primarily.) I read about using this in a paperback on how to detail cars and have been very satisfied with the results. It makes rubber hoses and plastic parts shine like new and does a great job of cleaning up oil residue and diesel film.

Others, on this forum, have mentioned biodiesel and kerosene. These would probably be good for major, gunky build-up but use them at a car wash. There they have sand traps to catch the residue, rather than contaminate ground water.

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Sam

84 300SD 350K+ miles ( Blue Belle )