Cummins ISX oil problem

I have an 06 Volvo with a Cummins ISX. My oil pressure has been running on the low end (about 20 idle/30ish running). I only had to add 1 gallon of oil every 15,000 miles. I got my oil changed on 11/14. I drove about 400 miles last night and by the top I stopped my running oil pressure was at about 28/29 psi. I started back home 400 miles and got about 70 miles away and my low pressure warning came on and I checked oil and was down 2 gallons. Added the oil and it went back up to 30ish. I have driven 5,500 miles since the oil change. I took it to a mechanic and he said I needed inframe (didn't look at it just taking a hunch). Mainly because there wasn't anywhere he could visibly see where the oil was going. He said it was probably coming out my stack and to look and see if I had a shiny black gloss on my trailer where the stack is. I looked and didn't. Is there anything else it could be that could be causing my issue or is inframe only solution and do I have anyway in finding out? My engine has 820,000 miles on it so I knew it was coming but I was hoping to get a little more out of it. It's sucks bc it's been running like a charm. Sorry so long and thanks for all input!!

 

No oil in the coolant. I checked my oil this morning and it was over full so instead of being down a gallon I was probably down a little over a gallon.

 

I have always used Rotella. To tell you the truth I'm not 100 percent on the oil now I'm getting engine chatter so I'm thinking my engine might be done. Crap this sucks.

 

It was a pulley that was making the noise wheewwww still keeping check on the oil.