|The rear wheel bearings are lubricated by the oil in the differential. This should be checked occasionally by opening the plug on the rear cover and seeing if the oil level sits at the bottom of the opening (when the car is level). If not, add an approved lubricant. If the differential lubricant is nasty, or if you suspect it to be contaminated, remove the rear cover to drain the entire system, then reinstall the cover with a new gasket, and refill the system. The shop manual shows the capacity as 3.0 US pints or 1.42 liters.
The rear wheel bearing assembly is sealed by an o-ring and the gasket for the flange plate / bearing retainer. You might be able to stop a minor seepage if you can (a) clean the area of all debris, (b) loosen the backing plate, which presses against the flange plate, (c) remove any remaining oil with an aerosol cleaner, (d) squirt some Permatex, Hylomar, or silicone sealer in the joints, then (e) reassemble. You don't have to drain the lubricant from the differential if you raise the work side of the car a couple of inches higher than the far side.
If that doesn't work, you'll have to drain the lubricant from the differential, pull the axle using an inertia puller (slide hammer), and replace the o-ring and gasket. This would also be the ideal time to replace the rear wheel bearings, if desired -- which requires the use of a really big press and is best done by a shop that's set up to do that sort of thing.
That said, I've never yet seen a case where rear end lubricant contaminated a braking system by leaking into it. I'd think it far more likely that what you found was actually brake fluid that hadn't been changed for ages -- the stuff looks and feels a lot like oil when it picks up rust. If there actually was a petroleum-based substance mixed with the brake fluid, it can damage or destroy even modern 'rubber' seals within hours. For safety's sake the entire brake system including metal lines should be (a) disassembled and removed from the car, (b) flushed with copious amounts of denatured alcohol followed by aerosol brake cleaner, (c) allowed to dry, then (d) rebuilt with all new 'rubber'. (This means installing new kits for the calipers, rear wheel cylinders, and master cylinder; refurbishing the PDWA; and replacing all three hoses. You may be able to avoid replacing the hoses if they are new or nearly so, and are proven by visual examination to be free from internal cracks. You don't have to replace the hoses at all, if they are teflon lined.)