DT466E with disappearing oil

2003 International 4700 dt466e. On a 2 hour trip yesterday the low oil pressure warning went off. I immediately pulled over and shut it down. No oil on stick! I had checked it that morning and was fine. After putting 4 gallons of oil back in it I was on my way. Another 30 minutes home and it was already a gallon low again. It's running perfect, and no smoke. The coolant is clean with no oil. Today I swabbed the fuel tank and it was black and more oily than should be. Also 6 months ago, we had a serious oil leak down the drivers side of the block and I pulled the high pressure fuel manifold and replaced the gasket. That fixed my oil leak and I had forgotten about it till today, but could my disappearing oil be getting into my fuel through an internal leak behind that manifold or is there something else to check that maybe common on this engine? I'm assuming it could be injector o rings also? But like I said it's running great as usual no smoke. Help or ideas appreciated. Thanks

 

Yes, I'm definitely figuring that it's

 

Look in my tanks for what? I already said there was oil in the tanks. No injector replacement recently. We I framed this truck around 125,000 miles ago.