7.8 isuzi duramax has engine oil in fuel tank

[flht1550]

I have a 2004 Isuzu 7.8 inline losing engine oil and not knowing why. I was changing the fuel filters on this truck and found the fuel was black in the filters. So I feel that this is where my engine oil is going. I've asked other people that I know and they all seem very surprised at this and have never heard of this. Hoping someone has ran into this before and can point me in the correct direction to correct this issue