Oh my!

Keep your cousin away from the car.

Just drain and refill with oil. If you are really worried you can run some motor flush through the engine.

While ATF has MUCH different properties than oil, the residue will NOT be a problem.

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1981 300D 147k
1998 VW Jetta Tdi 320k
2001 Dodge Ram 2500 141k
1979 300D 234k (sold)
1984 300D "Astor" 262k(sold)
Mercedes How-To and Repair Pictorials
I love the smell of diesel smoke in my hair