Rear Seal, Or Oil Pan Leak??

I'd throw a new rear seal in just because it's right there, clean it up really good, loosen the rear-most pan bolts, scrape out what gasket I could easily remove, and squirt some 'Right Stuff' gasket maker in there, and tighten. That's super-sketchy but it won't hurt anything, and might buy you some time until you have the engine out. I swear by 'Right Stuff' since it permanently fixed the yearly leak in my Cub Cadet mower's (vertical) valve cover gasket that no amount of regular RTV would fix.

Engine oil weight won't really help the leak and might cause other problems (if you're running something too thick when it's cold out). ATP AT-205 stop leak helped some dried out seals in my old Windstar and a rope-seal in an old 289. Might help if your pan gasket is just dried out. Gets pretty good reviews, but it's always hit-or-miss with sealer.

I sound like an informercial, but that's what I'd do short of pulling the engine.