When I looked at Ron`s "Pandora`s Box" about the gunk acting as a self-made "BandAid" on a seal, it would make sense in some situations.
Working in mechanical design I know there are SO MANY different seal designs (single lip, double lip, spring-loaded, etc) and material types (Butyl Rubber, Viton, silicone, felt, etc) used for seals.
Most seals fail for a number of reasons:
1) Wear from friction OR environmental elements, like dirt or paper dust or chemical process fumes (most common)
2) Age (especially rubber)
3) Incompatibility with fluid or gas trying to be sealed (like gasoline with neoprene rubber)
4) Wrong seal design or wrong tolerances on housing for seal for application (happens when engineer "just picks one" or "copies" previous (bad)design for new design)
5) Improper installation by mechanic/service tech (hammer and "right sized" socket being a common method instead of correct special tool and arbor press)
Any one who has worked on or serviced cars or machinery for any length of time has experienced one of these situations. In your situation, I suspect it`s age, even with the lower mileage.
Engineers and designers get the blame for a lot of seal problems, and rightly so when you look at the list above. However, a really good seal may be deemed too expensive an option during a cost analysis design review. It always pained me when this was done. It`s the "penny wise/pound foolish" approach, BUT If (and I repeat, IF) it lasts as long as warranty of the vehicle or machine, then using the cheaper seal is the "fiscally correct" solution. With the advent of computer design and material simulation analysis programs, engineers can "design" products for just that length of warranty time. It`s called "design obsolescence" and it`s an integral part of every manufacturing business economy and product you buy today. It`s also the reason why the energy-efficient refrigerator you bought for your house 5 years ago is on the fritz while the old Kelvenator fridge your grandfather bought in 1962 that you inherited and is now in the garage storing your carnauba waxes,welding rods, fishing bait (wax worms), and "favorite beverages" is still running strong and cold!
Sorry to rant and rave about this off topic.... just had to vent.