Are you sure it's engine oil, and not A/C refrigerant?

I'm not all that knowledgeable about how all this stuff is routed, but the A/C evaporator is located somewhere behind the dash, right?

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1987 Mercedes 300D ~200K (Greasecar & Biodiesel)
1993 Ford F-250 7.3 IDI diesel 165K (Biodiesel)
1996 Thomas/International Bus with DT466 engine