Went thru a similar dilemma myself and ended up using Marvel Mystery Oil. I just put it in Friday night so we'll see how it goes...but, with your rear main seal leak, you might want to use Auto RX...
Another thought, I had a front seal leak when I first got my truck, I used engine cleaner and sprayed the grime away to have a cleaner working area to replace the seal and to my surprise, the seal all but stopped leaking..it now only seeps a little! Evidently there was some grime deforming the seal that was washed away and it sealed back up...I said all that to say that maybe using MMO first might wash out any grime on your rear seal since it will thin the oil a little..or, it could just leak more and not clean out anything, in which case I would go with the ARX at the next oil change.
This Kreen seems like it is some good stuff, but most of my reading about these engines say that they stay pretty darn clean with regular oil changes. How much Kreen would be recommended with our oil capacity..a quart? maybe two?...added to 13 qts of 15w40 shouldn't thin it past a 30 wt...and it might do the same and wash the seal off...but it is a little costlier...
I went with MMO to do a little cleaning and reduce the viscosity of my 15w40 a little for the cooler weather. I wasn't due for an oil change so I just dropped the oil filter, drained it, added 1 qt MMO in the filter and replaced it, started the motor for about a minute, shut it off, waited a few minutes and added MMO to get it to a good level - which ended up being 2.5 qts of MMO.
I got a lot of opinions saying nothing was needed except regular 5k oil changes to keep these engines clean...even got pics of engines opened for one reason or another that showed them to be real clean that had only had regular oil changes...so, chances are, our engines are fairly clean inside...your 4K OCIs should surely be good for keeping one pretty clean and the guy I bought mine from said he had done 3k oil changes (second owner but only 162K). I don't think a little extra cleaning will hurt especially if done during the colder months to compensate for the reduced viscosity so the injectors should run fine...???
