2011 Peterbilt 387, Rear Main Seal Leak??

My truck has been leaking oil lately... it's a automatic.. it started when I had to move my tandems with 43k of mulch in the box. I've been watching it closely. It's a slow leak. I have to add regularly. I stopped in a TA to have my Trans fluid checked, it was full well over full. Asked the mechanic to see where the leak was coming from and he said it's the rear main seal. When running 64.6 mph oil pressure is at 30- 32.5- 35, at idle the oil pressure is at 15. If I high idle it it's at 25. I know 20 is normal pressure at least the mannual says so. Any thoughts? Also the oil pressure goes low when a oil change is needed. Oil has been changed.

 

Ok thanks! If it is the seal Def isn't a small task .... I'll take that plate off and see what I see. That oil pan gasket does look pretty bad. Overall it's a good truck... Great mpg!

 

The leak is coming from the front. That's gonna cost about $1,300! Yes!