I am always puzzled that we don't see this question more often. The two are likely very closely related, sludging and rear main seepage. If you haven't already done so, check out the PCV valve and associated plumbing to be sure it is operational. This could have been the cause of the sludging as well as the leak. Restoring the PCV valve to operationality if this is the case could drastically slow down the leak on its own. Also if the valve is stuck, sludging will likely occur again once we get the motor cleaned up.
I would start out by running a basic application. That is adding a full bottle of Auto-RX to a fresh fill of conventional oil, with a new filter in place. Operate the truck as you normally would for 2500 miles. Also I suggest changing out the filter only at the half way point or roughly 1250 miles. This will prevent the possibility of the filter becoming full and going into the bypass mode. After finishing up the 2500 mile run, drain the oil, change out the filter, and refill with conventional oil. Operate this oil change for 3000 miles. This is the rinse phase and no Auto-Rx is added during the rinse phase.
There is a good possibility that the rear main will be cured or slowed dramatically by the end of this first application. With respect to the top end of the motor, you will have made a good dent in the heavy sludge. At least the key components cam lobes, lifters, rockers, should be cleaned up greatly. Cosmetic areas that see only minor splash fed lubrication will take longer to clean up. My recommendation is to start the second application at this point. It will take two applications minimum to get the top end back to respectability.
Along the way you should get better ring to cylinder bore sealing, preventing a good deal of blow gases slipping past the rings. This will help take some pressure off the PCV system and prevent fuel dilution of the oil. Keep us abreast your progress.
... don't panic if your leak worsens during the treatment.
Sludge has caused your seal to lose its shape and now the seal no longer mates perfectly with the metal surrounding it, which is why you have a leak. Right now (before your treatment) sludge is actually helping to fill this gap between the seal and metal. When that sludge is removed, oil now has an even bigger gap to leak from, which can make the leak worse. Over time the leak should stop as the seal regains its original shape, but with a seal that old, it may not completely return to its original shape. If that happens you can either live with the leak, or replace the seal. The leak may not go away until you are in your second rinse phase, so be patient.
It is very important to run plain jane conventional oil in correcting rotary seal leaks. That means no gimick high mileage oils that claim to swell seals. insulting seals and making them pliable will make things worse. Plain mineral oil will help the seal find its original memory once the crud is dissolved off the crank journal and what is packed infront and likely even behind the seal. The good news is that the journal ends get good oil circulation. Areas of the motor that clean quickest would be the crank and the cylinder bores and ring packs. Sometimes it takes two rinse phases with conventional oil to get a leak completely stopped. However, in your case, I would just run the one rinse and go onto application number two, which will likely be working mainly on the top end of the motor, which would be my main concern. It is highly likely that the leak will have been minimized by then. Plus you will be gravity feeding dirty oil out of the top end during the clean and rinse and am sure that the counter balances on the crank are going to splash dirty oil outward towards the journals.
