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Diesel in oil

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Hi everyone. I have a 2002 Mack RD688S with E 7, 350 HP with 621,0047 miles on it. Yesterday when I checked the oil I noticed it appeared to have diesel in the oil. The truck doesn't miss or smoke. So what could be causing this?

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Yup oilly! also fairly common as strange as it may seem  is injector orings fuel can leak up by the injector hold down threads  ! and the fuel transfer pump  shaft seal can leak another semi common issue!

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Fuel transfer pump can leak fuel into the oil, there is an fuel galley plug under the front cover that can leak fuel into the oil as well. Usually there is fuel weeping around the fuel galley plugs on the front cover if the plug(s) are leaking. Best thing to do is remove the oil pan, use the fuel hand primer pump to charge the fuel system and try to find where its coming from. 

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