The bottom line is that the seal dried out from sitting, then tore when the engine was pressed back into service.  Running heavier weight oil isn't going to solve the issue; it might make the leak somewhat less, but at what expense?  Straight 40W oil will be like taffy when the engine is cold.  It will place undue strain on the oil pump and oil pump driveshaft (which I've seen twisted like a swizzel stick in a SBF before BTW). 

Seal repair in a can?  No thanks.  MS's story aside all that junk is is a bunch of stuff that swells the seals up.  It may work temporarily, but eventually the seal will fail catastrophically and instead of a little leak you end up with it dumping your oil everywhere.  Those products are basically made for someone who's selling the vehicle and doesn't want to go to the expense of fixing it right.

Do yourself a favor and just change the seal.  Its not really that big a deal (I just did mine about a month ago).  Drop the pan, pull the rear main cap, pull the oil pump, loosen all the mains, drive/yank the old upper part of the seal out of the block, clean any sealer off the cap & block, slide in the new upper portion while turning the crank in the direction of installation, a little silicone on the cap mating surface & the seal ends (offset the seal about 3/8" so the ends don't line up with the parting line between the block & cap), and reinstall everything.

The biggest trick to it is to make up a guide tool so you don't shave the back of the new upper half of the seal on installation.  I just took a plastic water bottle and cut a strip of the thin, tough plastic to fit the groove in the block.  This provides protection for the seal to slide past, so it doesn't catch the edge of the groove in the block and shave itself.

Use some white lithium grease on the seal lips and crank, the back of the seals must be installed dry.

Oh, it could.  I've seen several engines destroyed from running the wrong weight oil.  Granted, those were newer engines (OHC, etc.), and these older engines are less precise, and likely would survive, but why risk it?  When an engine is designed the oil system is designed to run a certain weight oil.  This ensures proper lubrication and pressure.

Race engines I'm seeing now are running continually lighter weight oils, because with synthetics you don't need the viscosity.  In claimer classes of drag racing guys run 0W20, and only two quarts at that.  The pan is literally dry by mid track in the 1/4, yet the engines survive like that for an entire year of racing because the oil is so good. 

Id did my rear seal on my 302 3 times using TKO's method and it leaked every time.  My next method is let it leak and fix it when I pull the engine and put a new one in...LOL   Steve69

Grabber Blu wrote:

The problem with the 2 piece rear main seal is they stopped making the seal that worked the best- Felpro 2902 that is a tan color.  From  what I understand the Ford replacement was like the Felpro 2902, which is no longer available.
The Felpro BS 40042 black one is too hard.
Cometic C5689 too soft
SBC 13605 not sure but leaks
I was able to find NOS felpro 2902 but haven't installed yet, maybe next winter

They don't  leak bad but always a drop of oil on the bell

The best solution is to have the block machined for a one piece seal
 

I used an old Felpro 2902 I has still in the package on the shelf, which explains why mine seems to have stopped the leak.  I also think a lot of it is in the installation.  They used to give you the little tool I had to make with the seal, but stopped doing that.  If you try to install one without that tool it will 100% certainly shave the back of the seal and it will leak. 

My understanding is that synthetic molecules are all the same size .... if one wedges under the lip of the seal, another can slide past it. 
Dino oil, with its varying size molecules, can pack against the seal, thus sealing it.
Simplified so that I can understand it. 
I have been wrong before tho.

Thanks for all your input on the rear main seal problem.  I've heard that this 2 piece seal was a rope seal while others tell me its a neoprene seal, which is it?
Just to be clear, I used regular dino 10W30 oil on the oil change.  Before I changed out the old Texaco Havoline, the rear main was not leaking nearly as bad.

Depends, it could be either.  The stock seal was a rope seal.  The rope seal could have been replaced with another rope seal, or a rubber seal at some point in the engine's life.  Only one way to know for sure...